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Chapter 31 ■ Chromosomal Abnormalities 1137
A
B
C D
FIGURE 31-12. Sonographic findings in trisomy 13. A, Alobar holoprosencephaly at 11 weeks. B, Proboscis (arrow).
C, Postaxial polydactyly on 3-D image at 14 weeks. D, 3-D scan of a third-trimester fetus shows a large, midline facial cleft.
TRISOMY 13:
SONOGRAPHIC FINDINGS
Holoprosencephaly
Microcephaly
Neural tube defects
Facial clefts
Ocular anomalies
Cardiac defects
Echogenic intracardiac focus
Cystic hygroma
Postaxial polydactyly
Echogenic kidneys
Intrauterine growth restriction
mation, and other abnormalities of the posterior fossa.
Alobar holoprosencephaly is often associated with
severe midline facial defects, including hypotelorism,
microphthalmia, and cyclopia. In addition, fetuses with
trisomy 13 often have postaxial polydactyly, abnormal
hand configuration, echogenic kidneys (30%), atypical
calcifications,
170-174
and IUGR.
172
TRIPLOIDY
Triploidy is the result of a complete extra set of chromosomes (69 chromosomes) and is not related to maternal
175
The extra set of chromosomes is often paternally
age.
1138 PART IV ■ Obstetric Sonography
A
B C
D E F
FIGURE 31-13. Sonographic findings in triploidy. A, Dandy-Walker malformation (arrow). B and C, Asymmetrical growth
restriction. Note the discrepancy in size of the head and body. D, Syndactyly of the fingers. E, Scan through the placenta showing multiple
lucencies as well as an omphalocele (arrow). F, Maternal ovaries with multiple cysts.
derived (73%) and usually occurs from a double fertilization. Less often, triploidy results from fertilization of a
diploid egg.
tions, and most are spontaneously aborted.
176
Triploidy occurs in 1% to 3% of concep-
177,178
The
prevalence of triploidy between 16 and 20 weeks is
1 : 5000 pregnancies, and survival of a fetus with triploidy
beyond 20 weeks’ gestation is unusual.
175
Fetuses
with triploidy surviving into the midtrimester have
a multitude of structural malformations, most often
involving the CNS, heart, and hands, as well as marked
asymmetrical growth restriction
178,179
(Fig. 31-13).
associated with a small placenta and IUGR.
loidy can be associated with maternal complications,
including early-onset preeclampsia, bilateral multicystic
ovaries, hyperemesis gravidarum, and persistent trophoblastic disease.
Jauniaux et al.
180,181
178
described 70 cases of triploidy
scanned between 13 and 29 weeks’ gestation. Anatomic
defects were found in 93% of cases, with abnormalities
of the hands (predominantly 3-4 syndactyly) the most
frequent finding (52%). Cerebral ventriculomegaly
was identified in 37%. Cardiac defects were detected in
176,177
Trip-
34% of fetuses, primarily atrioventricular septal defects.
Micrognathia affected 26% of fetuses. Placental molar
TRIPLOIDY: SONOGRAPHIC FINDINGS
3-4 Syndactyly
Cardiac defects
Neural tube defects
Posterior fossa anomalies
Heart defects
Cystic hygroma
Asymmetric growth restriction
Abnormal placenta
Oligohydramnios
Renal anomalies
Omphalocele
changes were seen in 29%, and amniotic fluid volume
was decreased in 44%. Asymmetrical growth restriction
was noted in 72% of cases, and each of these fetuses had
a sonographically normal–appearing placenta.
178
TURNER (45,X) SYNDROME
Turner syndrome is the result of a 45,X chromosomal
complement, usually caused by loss of the paternal X
chromosome, and is unrelated to maternal age. About
95% of conceptuses are spontaneously aborted. Turner
syndrome occurs in 1 : 2000 to 1 : 5000 live births.
182-185
The lethal type of Turner syndrome seen in the midtri-
Paternal triploid origin is associated with a large placenta filled with cystic spaces. Triploidy on the basis of
an extra maternal chromosomal complement is usually
mester of pregnancy generally presents with large septated cystic hygromas, total body lymphedema, pleural
effusions, ascites, and cardiac defects
182,183
(Fig. 31-14).
A B
Chapter 31 ■ Chromosomal Abnormalities 1139
C D
FIGURE 31-14. Sonographic findings in Turner (45,X) syndrome. A and B, Large septated cystic hygromas. C, Hydropic
fetal arm in a fetus with severe lymphangiectasia. D, Small aorta (arrow) in fetus with interrupted aortic arch.
Cystic hygromas are malformations of the lymphatic
system and appear as saccular septated fluid collections,
most often surrounding the back of the fetal head
and neck. Azar et al.
184
reported that 75% of fetuses
with bilateral dorsal septated nuchal cervical cystic
hygromas had chromosomal anomalies, the most
common being Turner syndrome (94%). Although
many second-trimester fetuses with cystic hygromas have
Turner syndrome, other karyotypic abnormalities,
including trisomies 21, 18, and 13 and triploidy, have
also been reported. In general, cystic hygromas in fetuses
with Turner syndrome are larger than those seen with
other karyotypic abnormalities, and Turner fetuses may
also have generalized lymphedema extending down the
torso and extremities.
Cardiac abnormalities, most often left-sided defects
such as coarctation of the aorta, may be identified in
10% to 48% of fetuses with Turner syndrome. However,
this may be an underestimation because many fetuses
are identified late in the first semester or early in the
second trimester, when optimal cardiac evaluation is not
likely.
182-185
Baena et al.
182
reported on 125 cases of
Turner syndrome from an unselected population and
noted that 67% were identified prenatally. The most
1140 PART IV ■ Obstetric Sonography
common sonographic findings were cystic hygromas
(59%) and hydrops (19%).
CONCLUSION
Over the last two decades, risk assessment for aneuploidy
has been refined to the point that maternal age alone is
no longer considered adequate in determining the risk
of having a chromosomally abnormal offspring. Obstetric sonography, in conjunction with serum analysis, has
become a powerful tool in the assessment of risk for
aneuploidy, in both the first and the second trimester.
In the midtrimester the diverse sonographic patterns
seen in the different aneuploidies allows clinicians to
guide patients to a presumptive diagnosis. The information obtained noninvasively helps the expectant couple
to weigh the risks of invasive testing against the probability of having a child with an abnormality. The goal
of screening is the detection of a greater number
of karyotypically abnormal fetuses with fewer invasive
procedures and subsequently the loss of fewer normal
fetuses.
References
1. ACOG Practice Bulletin No. 77. Screening for fetal chromosomal
abnormalities. Obstet Gynecol 2007;109:217-227.
2. Peller AJ, Westgate MN, Holmes LB. Trends in congenital malformations, 1974-1999: effect of prenatal diagnosis and elective termination. Obstet Gynecol 2004;104:957-964.
3. Egan JF, Benn P, Borgida AF, et al. Efficacy of screening for fetal
Down syndrome in the United States from 1974 to 1997. Obstet
Gynecol 2000;96:979-985.
4. Schreinemachers DM, Cross PK, Hook EB. Rates of trisomies 21,
18, 13 and other chromosome abnormalities in about 20,000 prenatal studies compared with estimated rates in live births. Hum
Genet 1982;61:318-324.
5. Resta RG. Changing demographics of advanced maternal age (AMA)
and the impact on the predicted incidence of Down syndrome in
the United States: implications for prenatal screening and genetic
counseling. Am J Med Genet A 2005;133A:31-36.
6. Snijders RJ, Sebire NJ, Nicolaides KH. Maternal age and gestational
age–specific risk for chromosomal defects. Fetal Diagn Ther
1995;10:356-367.
7. Nicolaides KH. Nuchal translucency and other first-trimester
sonographic markers of chromosomal abnormalities. Am J Obstet
Gynecol 2004;191:45-67.
First-Trimester Screening for Aneuploidy
8. Langdon JLH. Observations on an ethnic classification of idiots. In
Clinical Lectures and Reports; 1866:259.
9. Nicolaides KH, Azar G, Byrne D, et al. Fetal nuchal translucency:
ultrasound screening for chromosomal defects in first trimester of
pregnancy. BMJ 1992;304:867-869.
10. Bekker MN, Haak MC, Rekoert-Hollander M, et al. Increased
nuchal translucency and distended jugular lymphatic sacs on firsttrimester ultrasound. Ultrasound Obstet Gynecol 2005;25:
239-245.
11. Souka AP, von Kaisenberg CS, Hyett JA, et al. Increased nuchal
translucency with normal karyotype. Am J Obstet Gynecol 2005;
192:1005-1021.
12. Pandya PP, Snijders RJ, Johnson SP, et al. Screening for fetal trisomies by maternal age and fetal nuchal translucency thickness at
10 to 14 weeks of gestation. Br J Obstet Gynaecol 1995;102:
957-962.
13. Snijders RJ, Noble P, Sebire N, et al. UK multicentre project on
assessment of risk of trisomy 21 by maternal age and fetal nuchaltranslucency thickness at 10-14 weeks of gestation. Fetal Medicine
Foundation First Trimester Screening Group. Lancet 1998;352:
343-346.
14. Wald NJ, Kennard A, Hackshaw AK. First trimester serum screening
for Down’s syndrome. Prenat Diagn 1995;15:1227-1240.
15. Brizot ML, Snijders RJ, Bersinger NA, et al. Maternal serum pregnancy-associated plasma protein A and fetal nuchal translucency
thickness for the prediction of fetal trisomies in early pregnancy.
Obstet Gynecol 1994;84:918-922.
16. Brizot ML, Snijders RJ, Butler J, et al. Maternal serum hCG and
fetal nuchal translucency thickness for the prediction of fetal trisomies in the first trimester of pregnancy. Br J Obstet Gynaecol
1995;102:127-132.
17. Wald NJ, Kennard A, Hackshaw A, McGuire A. Antenatal screening
for Down’s syndrome. J Med Screen 1997;4:181-246.
18. Spencer K, Spencer CE, Power M, et al. Screening for chromosomal
abnormalities in the first trimester using ultrasound and maternal
serum biochemistry in a one-stop clinic: a review of three years
prospective experience. BJOG 2003;110:281-286.
19. Wapner R, Thom E, Simpson JL, et al. First-trimester screening for
trisomies 21 and 18. N Engl J Med 2003;349:1405-1413.
20. Wald NJ, Rodeck C, Hackshaw AK, et al. First and second trimester
antenatal screening for Down’s syndrome: the results of the Serum,
Urine and Ultrasound Screening Study (SURUSS). J Med Screen
2003;10:56-104.
21. Malone FD, Canick JA, Ball RH, et al. First-trimester or secondtrimester screening, or both, for Down’s syndrome. N Engl J Med
2005;353:2001-2011.
22. Sebire NJ, Snijders RJ, Hughes K, et al. Screening for trisomy 21 in
twin pregnancies by maternal age and fetal nuchal translucency
thickness at 10-14 weeks of gestation. Br J Obstet Gynaecol
1996;103:999-1003.
23. Kagan KO, Gazzoni A, Sepulveda-Gonzalez G, et al. Discordance
in nuchal translucency thickness in the prediction of severe twin-totwin transfusion syndrome. Ultrasound Obstet Gynecol 2007;29:
527-532.
24. Spencer K, Nicolaides KH. Screening for trisomy 21 in twins using
first trimester ultrasound and maternal serum biochemistry in a
one-stop clinic: a review of three years experience. BJOG 2003;110:
276-280.
25. Comstock CH, Malone FD, Ball RH, et al. Is there a nuchal translucency millimeter measurement above which there is no added
benefit from first trimester serum screening? Am J Obstet Gynecol
2006;195:843-847.
26. Wald NJ, Watt HC, Hackshaw AK. Integrated screening for Down’s
syndrome on the basis of tests performed during the first and second
trimesters. N Engl J Med 1999;341:461-467.
27. Palomaki GE, Steinort K, Knight GJ, Haddow JE. Comparing three
screening strategies for combining first- and second-trimester Down
syndrome markers. Obstet Gynecol 2006;107:367-375.
28. Cuckle HS, Malone FD, Wright D, et al. Contingent screening for
Down syndrome: results from the FASTER trial. Prenat Diagn
2008;28:89-94.
29. Ball RH, Caughey AB, Malone FD, et al. First- and second-trimester
evaluation of risk for Down syndrome. Obstet Gynecol 2007;110:
10-17.
30. Platt LD, Greene N, Johnson A, et al. Sequential pathways of testing
after first-trimester screening for trisomy 21. Obstet Gynecol
2004;104:661-666.
31. Haddow JE, Palomaki GE, Knight GJ, et al. Screening of maternal
serum for fetal Down’s syndrome in the first trimester. N Engl J
Med 1998;338:955-961.
32. Snijders RJ, Thom EA, Zachary JM, et al. First-trimester trisomy
screening: nuchal translucency measurement training and quality
assurance to correct and unify technique. Ultrasound Obstet
Gynecol 2002;19:353-359.
33. Evans MI, van Decruyes H, Nicolaides KH. Nuchal translucency
measurements for first-trimester screening: the ‘price’ of inaccuracy.
Fetal Diagn Ther 2007;22:401-404.
34. Braithwaite JM, Kadir RA, Pepera TA, et al. Nuchal translucency
measurement: training of potential examiners. Ultrasound Obstet
Gynecol 1996;8:192-195.
35. Nicolaides KH. The 11-13+6 weeks scan. London: Fetal Medicine
Foundation; 2004.
Chapter 31 ■ Chromosomal Abnormalities 1141
36. Wax JR, Pinette MG, Cartin A, Blackstone J. The value of repeated
evaluation after initial failed nuchal translucency measurement.
J Ultrasound Med 2007;26:825-828; quiz 829-830.
37. Breathnach FM, Malone FD, Lambert-Messerlian G, et al. First- and
second-trimester screening: detection of aneuploidies other than
Down syndrome. Obstet Gynecol 2007;110:651-657.
38. Spencer K, Nicolaides KH. A first trimester trisomy 13/trisomy 18
risk algorithm combining fetal nuchal translucency thickness, maternal serum free beta-hCG and PAPP-A. Prenat Diagn 2002;22:
877-879.
39. Malone FD, Ball RH, Nyberg DA, et al. First-trimester septated
cystic hygroma: prevalence, natural history, and pediatric outcome.
Obstet Gynecol 2005;106:288-294.
40. Molina FS, Avgidou K, Kagan KO, et al. Cystic hygromas, nuchal
edema, and nuchal translucency at 11-14 weeks of gestation. Obstet
Gynecol 2006;107:678-683.
41. Sonek J, Croom C, McKenna D, Neiger R. First-trimester septated
cystic hygroma: prevalence, natural history, and pediatric outcome.
Obstet Gynecol 2006;107:424; author reply 425.
42. Cicero S, Dezerega V, Andrade E, et al. Learning curve for sonographic examination of the fetal nasal bone at 11-14 weeks. Ultrasound Obstet Gynecol 2003;22:135-137.
43. Cicero S, Curcio P, Papageorghiou A, et al. Absence of nasal bone
in fetuses with trisomy 21 at 11-14 weeks of gestation: an observational study. Lancet 2001;358:1665-1667.
44. Cicero S, Rembouskos G, Vandecruys H, et al. Likelihood ratio for
trisomy 21 in fetuses with absent nasal bone at the 11-14–week scan.
Ultrasound Obstet Gynecol 2004;23:218-223.
45. Cicero S, Bindra R, Rembouskos G, et al. Integrated ultrasound and
biochemical screening for trisomy 21 using fetal nuchal translucency, absent fetal nasal bone, free beta-hCG and PAPP-A at 11 to
14 weeks. Prenat Diagn 2003;23:306-310.
46. Cicero S, Avgidou K, Rembouskos G, et al. Nasal bone in first-trimester screening for trisomy 21. Am J Obstet Gynecol 2006;195:109.
47. Sepulveda W, Wong AE, Dezerega V. First-trimester ultrasonographic screening for trisomy 21 using fetal nuchal translucency and
nasal bone. Obstet Gynecol 2007;109:1040-1045.
48. Malone FD, Ball RH, Nyberg DA, et al. First-trimester nasal bone
evaluation for aneuploidy in the general population. Obstet Gynecol
2004;104:1222-1228.
49. Rosen T, D’Alton ME, Platt LD, Wapner R. First-trimester ultrasound assessment of the nasal bone to screen for aneuploidy. Obstet
Gynecol 2007;110:399-404.
50. Ville Y. What is the role of fetal nasal bone examination in the
assessment of risk for trisomy 21 in clinical practice? Am J Obstet
Gynecol 2006;195:1-3.
51. Sonek J, Borenstein M, Dagklis T, et al. Frontomaxillary facial angle
in fetuses with trisomy 21 at 11-13(6) weeks. Am J Obstet Gynecol
2007;196:271 e1-e4.
52. Borrell A, Gonce A, Martinez JM, et al. First-trimester screening for
Down syndrome with ductus venosus Doppler studies in addition
to nuchal translucency and serum markers. Prenat Diagn
2005;25:901-905.
53. Falcon O, Auer M, Gerovassili A, et al. Screening for trisomy 21 by
fetal tricuspid regurgitation, nuchal translucency and maternal
serum free beta-hCG and PAPP-A at 11 + 0 to 13 + 6 weeks. Ultrasound Obstet Gynecol 2006;27:151-155.
54. Hyett J, Perdu M, Sharland G, et al. Using fetal nuchal translucency
to screen for major congenital cardiac defects at 10-14 weeks of
gestation: population based cohort study. BMJ 1999;318:81-85.
55. Makrydimas G, Sotiriadis A, Ioannidis JP. Screening performance of
first-trimester nuchal translucency for major cardiac defects: a metaanalysis. Am J Obstet Gynecol 2003;189:1330-1335.
56. Simpson LL, Malone FD, Bianchi DW, et al. Nuchal translucency
and the risk of congenital heart disease. Obstet Gynecol 2007;109:
376-383.
57. Michailidis GD, Economides DL. Nuchal translucency measurement and pregnancy outcome in karyotypically normal fetuses.
Ultrasound Obstet Gynecol 2001;17:102-105.
58. Souka AP, Krampl E, Bakalis S, et al. Outcome of pregnancy in
chromosomally normal fetuses with increased nuchal translucency
in the first trimester. Ultrasound Obstet Gynecol 2001;18:9-17.
59. Bilardo CM, Muller MA, Pajkrt E, et al. Increased nuchal translucency thickness and normal karyotype: time for parental reassurance.
Ultrasound Obstet Gynecol 2007;30:11-18.
60. Westin M, Saltvedt S, Almstrom H, et al. By how much does
increased nuchal translucency increase the risk of adverse pregnancy
outcome in chromosomally normal fetuses? A study of 16,260
fetuses derived from an unselected pregnant population. Ultrasound
Obstet Gynecol 2007;29:150-158.
61. Senat MV, Bussieres L, Couderc S, et al. Long-term outcome of
children born after a first-trimester measurement of nuchal translucency at the 99th percentile or greater with normal karyotype: a
prospective study. Am J Obstet Gynecol 2007;196:53 e1-e6.
Second-Trimester Screening for Trisomy 21
62. Vintzileos AM, Egan JF. Adjusting the risk for trisomy 21 on the
basis of second-trimester ultrasonography. Am J Obstet Gynecol
1995;172:837-844.
63. Bromley B, Lieberman E, Benacerraf BR. The incorporation of
maternal age into the sonographic scoring index for the detection at
14-20 weeks of fetuses with Down’s syndrome. Ultrasound Obstet
Gynecol 1997;10:321-324.
64. Bromley B, Shipp T, Benacerraf BR. Genetic sonogram scoring
index: accuracy and clinical utility. J Ultrasound Med 1999;18:523528; quiz 529-530.
65. Winter TC, Uhrich SB, Souter VL, Nyberg DA. The “genetic sonogram”: comparison of the index scoring system with the age-adjusted
ultrasound risk assessment. Radiology 2000;215:775-782.
66. Nyberg DA, Souter VL, El-Bastawissi A, et al. Isolated sonographic
markers for detection of fetal Down syndrome in the second trimester of pregnancy. J Ultrasound Med 2001;20:1053-1063.
67. Bromley B, Lieberman E, Shipp TD, Benacerraf BR. The genetic
sonogram: a method of risk assessment for Down syndrome in
the second trimester. J Ultrasound Med 2002;21:1087-1096; quiz
1097-1098.
68. Vintzileos AM, Guzman ER, Smulian JC, et al. Second-trimester
genetic sonography in patients with advanced maternal age and
normal triple screen. Obstet Gynecol 2002;99:993-995.
69. Hobbins JC, Lezotte DC, Persutte WH, et al. An 8-center study to
evaluate the utility of mid-term genetic sonograms among high-risk
pregnancies. J Ultrasound Med 2003;22:33-38.
70. Smith-Bindman R, Hosmer W, Feldstein VA, et al. Second- trimester
ultrasound to detect fetuses with Down syndrome: a meta-analysis.
JAMA 2001;285:1044-1055.
71. Benacerraf BR, Barss VA, Laboda LA. A sonographic sign for the
detection in the second trimester of the fetus with Down’s syndrome. Am J Obstet Gynecol 1985;151:1078-1079.
72. Benacerraf BR, Frigoletto Jr FD. Soft tissue nuchal fold in the second-trimester fetus: standards for normal measurements compared
with those in Down syndrome. Am J Obstet Gynecol 1987;157:
1146-1149.
73. Benacerraf BR, Laboda LA, Frigoletto FD. Thickened nuchal fold
in fetuses not at risk for aneuploidy. Radiology 1992;184:
239-242.
74. Benacerraf BR, Frigoletto Jr FD, Cramer DW. Down syndrome:
sonographic sign for diagnosis in the second-trimester fetus. Radiology 1987;163:811-813.
75. Crane JP, Gray DL. Sonographically measured nuchal skinfold thickness as a screening tool for Down syndrome: results of a prospective
clinical trial. Obstet Gynecol 1991;77:533-536.
76. Donnenfeld AE, Meister D, Allison J, et al. Interobserver variability
of sonographically determined second-trimester nuchal skinfold
thickness measurements. Ultrasound Obstet Gynecol 1995;5:
119-122.
77. Gray DL, Crane JP. Optimal nuchal skin-fold thresholds based on
gestational age for prenatal detection of Down syndrome. Am J
Obstet Gynecol 1994;171:1282-1286.
78. Grandjean H, Sarramon MF. Sonographic measurement of nuchal
skinfold thickness for detection of Down syndrome in the secondtrimester fetus: a multicenter prospective study. The AFDPHE
Study Group. Association Francaise pour le Depistage et la Prevention des Handicaps de l’Enfant. Obstet Gynecol 1995;85:103-106.
79. Watson WJ, Miller RC, Menard MK, et al. Ultrasonographic measurement of fetal nuchal skin to screen for chromosomal abnormalities. Am J Obstet Gynecol 1994;170:583-586.
80. Landwehr Jr JB, Johnson MP, Hume RF, et al. Abnormal nuchal
findings on screening ultrasonography: aneuploidy stratification on
the basis of ultrasonographic anomaly and gestational age at detection. Am J Obstet Gynecol 1996;175:995-999.
1142 PART IV ■ Obstetric Sonography
81. Lynch L, Berkowitz GS, Chitkara U, et al. Ultrasound detection of
Down syndrome: is it really possible? Obstet Gynecol 1989;73:
267-270.
82. Smith-Bindman R, Chu P, Goldberg JD. Second trimester prenatal
ultrasound for the detection of pregnancies at increased risk of Down
syndrome. Prenat Diagn 2007;27:535-544.
83. Borrell A, Costa D, Martinez JM, et al. Criteria for fetal nuchal
thickness cut-off: a re-evaluation. Prenat Diagn 1997;17:23-29.
84. Wapner RJ. Nuchal fold and nasal bone: how should we use them
in Down syndrome screening? Am J Obstet Gynecol 2008;199:
213-214.
85. Sonek JD, Nicolaides KH. Prenatal ultrasonographic diagnosis of
nasal bone abnormalities in three fetuses with Down syndrome. Am
J Obstet Gynecol 2002;186:139-141.
86. Sonek JD, Cicero S, Neiger R, Nicolaides KH. Nasal bone assessment in prenatal screening for trisomy 21. Am J Obstet Gynecol
2006;195:1219-1230.
87. Bromley B, Lieberman E, Shipp TD, Benacerraf BR. Fetal nose bone
length: a marker for Down syndrome in the second trimester.
J Ultrasound Med 2002;21:1387-1394.
88. Vintzileos A, Walters C, Yeo L. Absent nasal bone in the prenatal
detection of fetuses with trisomy 21 in a high-risk population.
Obstet Gynecol 2003;101:905-908.
89. Odibo AO, Sehdev HM, Dunn L, et al. The association between
fetal nasal bone hypoplasia and aneuploidy. Obstet Gynecol
2004;104:1229-1233.
90. Bunduki V, Ruano R, Miguelez J, et al. Fetal nasal bone length:
reference range and clinical application in ultrasound screening for
trisomy 21. Ultrasound Obstet Gynecol 2003;21:156-160.
91. Cicero S, Sonek JD, McKenna DS, et al. Nasal bone hypoplasia in
trisomy 21 at 15-22 weeks’ gestation. Ultrasound Obstet Gynecol
2003;21:15-18.
92. Cusick W, Provenzano J, Sullivan CA, et al. Fetal nasal bone length
in euploid and aneuploid fetuses between 11 and 20 weeks’
gestation: a prospective study. J Ultrasound Med 2004;23:
1327-1333.
93. Odibo AO, Sehdev HM, Stamilio DM, et al. Defining nasal bone
hypoplasia in second-trimester Down syndrome screening: does the
use of multiples of the median improve screening efficacy? Am J
Obstet Gynecol 2007;197:361e1-e4.
94. Odibo AO, Sehdev HM, Gerkowicz S, et al. Comparison of the
efficiency of second-trimester nasal bone hypoplasia and increased
nuchal fold in Down syndrome screening. Am J Obstet Gynecol
2008;199:281e1-e5.
95. Benoit B, Chaoui R. Three-dimensional ultrasound with maximal
mode rendering: a novel technique for the diagnosis of bilateral or
unilateral absence or hypoplasia of nasal bones in second-trimester
screening for Down syndrome. Ultrasound Obstet Gynecol 2005;
25:19-24.
96. Lockwood C, Benacerraf B, Krinsky A, et al. A sonographic screening method for Down syndrome. Am J Obstet Gynecol 1987;
157:803-808.
97. Benacerraf BR, Gelman R, Frigoletto Jr FD. Sonographic identification of second-trimester fetuses with Down’s syndrome. N Engl J
Med 1987;317:1371-1376.
98. Benacerraf BR, Cnann A, Gelman R, et al. Can sonographers reliably
identify anatomic features associated with Down syndrome in
fetuses? Radiology 1989;173:377-380.
99. Nyberg DA, Resta RG, Hickok DE, et al. Femur length shortening
in the detection of Down syndrome: is prenatal screening feasible?
Am J Obstet Gynecol 1990;162:1247-1252.
100. Shipp TD, Bromley B, Mascola M, Benacerraf B. Variation in fetal
femur length with respect to maternal race. J Ultrasound Med
2001;20:141-144.
101. FitzSimmons J, Droste S, Shepard TH, et al. Long-bone growth in
fetuses with Down syndrome. Am J Obstet Gynecol 1989;161:
1174-1177.
102. Benacerraf BR, Neuberg D, Frigoletto Jr FD. Humeral shortening
in second-trimester fetuses with Down syndrome. Obstet Gynecol
1991;77:223-227.
103. Benacerraf BR, Mandell J, Estroff JA, et al. Fetal pyelectasis: a
possible association with Down syndrome. Obstet Gynecol 1990;
76:58-60.
104. Corteville JE, Dicke JM, Crane JP. Fetal pyelectasis and Down
syndrome: is genetic amniocentesis warranted? Obstet Gynecol
1992;79:770-772.
105. Nyberg DA, Resta RG, Luthy DA, et al. Prenatal sonographic findings of Down syndrome: review of 94 cases. Obstet Gynecol
1990;76:370-377.
106. Bromley B, Doubilet P, Frigoletto Jr FD, et al. Is fetal hyperechoic
bowel on second-trimester sonogram an indication for amniocentesis? Obstet Gynecol 1994;83:647-651.
107. Dicke JM, Crane JP. Sonographically detected hyperechoic fetal
bowel: significance and implications for pregnancy management.
Obstet Gynecol 1992;80:778-782.
108. Nyberg DA, Resta RG, Mahony BS, et al. Fetal hyperechogenic
bowel and Down’s syndrome. Ultrasound Obstet Gynecol 1993;
3:330-333.
109. Scioscia AL, Pretorius DH, Budorick NE, et al. Second-trimester
echogenic bowel and chromosomal abnormalities. Am J Obstet
Gynecol 1992;167:889-894.
110. Sepulveda W, Sebire NJ. Fetal echogenic bowel: a complex scenario.
Ultrasound Obstet Gynecol 2000;16:510-514.
111. Roberts DJ, Genest D. Cardiac histologic pathology characteristic
of trisomies 13 and 21. Hum Pathol 1992;23:1130-1140.
112. Bromley B, Lieberman E, Laboda L, Benacerraf BR. Echogenic
intracardiac focus: a sonographic sign for fetal Down syndrome.
Obstet Gynecol 1995;86:998-1001.
113. Winter TC, Anderson AM, Cheng EY, et al. Echogenic intracardiac
focus in 2nd-trimester fetuses with trisomy 21: usefulness as a US
marker. Radiology 2000;216:450-456.
114. Shipp TD, Bromley B, Lieberman E, Benacerraf BR. The frequency
of the detection of fetal echogenic intracardiac foci with respect to
maternal race. Ultrasound Obstet Gynecol 2000;15:460-462.
115. Winn VD, Sonson J, Filly RA. Echogenic intracardiac focus: potential for misdiagnosis. J Ultrasound Med 2003;22:1207-1214; quiz
1216-1217.
116. Rotmensch S, Liberati M, Bronshtein M, et al. Prenatal sonographic
findings in 187 fetuses with Down syndrome. Prenat Diagn 1997;17:
1001-1009.
117. DeVore GR, Alfi O. The use of color Doppler ultrasound to identify
fetuses at increased risk for trisomy 21: an alternative for high-risk
patients who decline genetic amniocentesis. Obstet Gynecol
1997;90:187.
118. Abuhamad AZ, Kolm P, Mari G, et al. Ultrasonographic fetal iliac
length measurement in the screening for Down syndrome. Am J
Obstet Gynecol 1994;171:1063-1067.
119. Shipp TD, Bromley B, Lieberman E, Benacerraf BR. The iliac angle
as a sonographic marker for Down syndrome in second-trimester
fetuses. Obstet Gynecol 1997;89:446-450.
120. Bahado-Singh RO, Wyse L, Dorr MA, et al. Fetuses with Down
syndrome have disproportionately shortened frontal lobe dimensions
on ultrasonographic examination. Am J Obstet Gynecol 1992;167:
1009-1014.
121. Benacerraf BR, Harlow BL, Frigoletto Jr FD. Hypoplasia of the
middle phalanx of the fifth digit: a feature of the second trimester
fetus with Down’s syndrome. J Ultrasound Med 1990;9:389-
394.
122. Shimizu T, Salvador L, Hughes-Benzie R, et al. The role of reduced
ear size in the prenatal detection of chromosomal abnormalities.
Prenat Diagn 1997;17:545-549.
123. Lettieri L, Rodis JF, Vintzileos AM, et al. Ear length in second-trimester aneuploid fetuses. Obstet Gynecol 1993;81:57-60.
124. Gill P, Vanhook J, Fitzsimmons J, et al. Fetal ear measurements in
the prenatal detection of trisomy 21. Prenat Diagn 1994;14:
739-743.
125. Gupta JK, Cave M, Lilford RJ, et al. Clinical significance of fetal
choroid plexus cysts. Lancet 1995;346:724-729.
126. Bromley B, Lieberman R, Benacerraf BR. Choroid plexus cysts:
not associated with Down syndrome. Ultrasound Obstet Gynecol
1996;8:232-235.
127. Benacerraf BR, Neuberg D, Bromley B, Frigoletto Jr FD. Sonographic scoring index for prenatal detection of chromosomal abnormalities. J Ultrasound Med 1992;11:449-458.
128. Benacerraf BR, Nadel A, Bromley B. Identification of second-trimester fetuses with autosomal trisomy by use of a sonographic scoring
index. Radiology 1994;193:135-140.
129. Vintzileos AM, Campbell WA, Guzman ER, et al. Second-trimester
ultrasound markers for detection of trisomy 21: which markers are
best? Obstet Gynecol 1997;89:941-944.
130. Bahado-Singh RO, Oz AU, Kovanci E, et al. New Down syndrome
screening algorithm: ultrasonographic biometry and multiple serum
Chapter 31 ■ Chromosomal Abnormalities 1143
markers combined with maternal age. Am J Obstet Gynecol
1998;179:1627-1631.
131. Bahado-Singh RO, Deren O, Tan A, et al. Ultrasonographically
adjusted midtrimester risk of trisomy 21 and significant chromosomal defects in advanced maternal age. Am J Obstet Gynecol
1996;175:1563-1568.
132. Nyberg DA, Luthy DA, Resta RG, et al. Age-adjusted ultrasound
risk assessment for fetal Down’s syndrome during the second trimester: description of the method and analysis of 142 cases. Ultrasound
Obstet Gynecol 1998;12:8-14.
133. Vintzileos AM, Campbell WA, Rodis JF, et al. The use of secondtrimester genetic sonogram in guiding clinical management of
patients at increased risk for fetal trisomy 21. Obstet Gynecol
1996;87:948-952.
134. Nyberg DA, Luthy DA, Cheng EY, et al. Role of prenatal ultrasonography in women with positive screen for Down syndrome on
the basis of maternal serum markers. Am J Obstet Gynecol
1995;173:1030-1035.
135. Vintzileos AM, Guzman ER, Smulian JC, et al. Choice of secondtrimester genetic sonogram for detection of trisomy 21. Obstet
Gynecol 1997;90:187-190.
136. Souter VL, Nyberg DA, Benn PA, et al. Correlation of secondtrimester sonographic and biochemical markers. J Ultrasound Med
2004;23:505-511.
137. Pinette MG, Egan JF, Wax JR, et al. Combined sonographic and
biochemical markers for Down syndrome screening. J Ultrasound
Med 2003;22:1185-1190.
138. DeVore GR, Romero R. Genetic sonography: an option for women
of advanced maternal age with negative triple-marker maternal
serum screening results. J Ultrasound Med 2003;22:1191-1199.
139. Aagaard-Tillery KM, Malone FD, Nyberg DA, et al. Role of secondtrimester genetic sonography after Down syndrome screening.
Obstet Gynecol 2009;114:1189-1196.
140. Rozenberg P, Bussieres L, Chevret S, et al. Screening for Down
syndrome using first-trimester combined screening followed by
second-trimester ultrasound examination in an unselected population. Am J Obstet Gynecol 2006;195:1379-1387.
141. Krantz DA, Hallahan TW, Macri VJ, Macri JN. Genetic sonography
after first-trimester Down syndrome screening. Ultrasound Obstet
Gynecol 2007;29:666-670.
Trisomy 18 (Edwards Syndrome)
142. Hook EB, Woodbury DF, Albright SG. Rates of trisomy 18 in
livebirths, stillbirths, and at amniocentesis. Birth Defects Orig Artic
Ser 1979;15:81-93.
143. Yamanaka M, Setoyama T, Igarashi Y, et al. Pregnancy outcome
of fetuses with trisomy 18 identified by prenatal sonography and
chromosomal analysis in a perinatal center. Am J Med Genet A
2006;140:1177-1182.
144. Jones KL. Smith’s recognizable patterns of human malformation, 5th
ed. Philadelphia: Saunders; 1997.
145. Papp C, Ban Z, Szigeti Z, et al. Role of second trimester sonography
in detecting trisomy 18: a review of 70 cases. J Clin Ultrasound
2007;35:68-72.
146. Watson WJ, Miller RC, Wax JR, et al. Sonographic findings of
trisomy 18 in the second trimester of pregnancy. J Ultrasound Med
2008;27:1033-1038; quiz 1039-1040.
147. Benacerraf BR, Harlow B, Frigoletto Jr FD. Are choroid plexus cysts
an indication for second-trimester amniocentesis? Am J Obstet
Gynecol 1990;162:1001-1006.
148. Nyberg DA, Kramer D, Resta RG, et al. Prenatal sonographic
findings of trisomy 18: review of 47 cases. J Ultrasound Med
1993;12:103-113.
149. Goetzinger KR, Stamilio DM, Dicke JM, et al. Evaluating the incidence and likelihood ratios for chromosomal abnormalities in fetuses
with common central nervous system malformations. Am J Obstet
Gynecol 2008;199:285e1-e6.
150. Carlson DE, Platt LD, Medearis AL. The ultrasound triad of fetal
hydramnios, abnormal hand posturing, and any other anomaly predicts autosomal trisomy. Obstet Gynecol 1992;79:731-734.
151. Benacerraf BR, Saltzman DH, Estroff JA, Frigoletto Jr FD. Abnormal karyotype of fetuses with omphalocele: prediction based on
omphalocele contents. Obstet Gynecol 1990;75:317-319.
152. Nicolaides KH, Salvesen DR, Snijders RJ, Gosden CM. Strawberryshaped skull in fetal trisomy 18. Fetal Diagn Ther 1992;7:
132-137.
153. Nyberg DA, Mahony BS, Hegge FN, et al. Enlarged cisterna magna
and the Dandy-Walker malformation: factors associated with chromosome abnormalities. Obstet Gynecol 1991;77:436-442.
154. Hill LM, Marchese S, Peterson C, Fries J. The effect of trisomy 18
on transverse cerebellar diameter. Am J Obstet Gynecol 1991;165:
72-75.
155. Thurmond AS, Nelson DW, Lowensohn RI, et al. Enlarged cisterna
magna in trisomy 18: prenatal ultrasonographic diagnosis. Am J
Obstet Gynecol 1989;161:83-85.
156. Shuangshoti S, Roberts MP, Netsky MG. Neuroepithelial (colloid)
cysts: pathogenesis and relation to choroid plexus and ependyma.
Arch Pathol 1965;80:214-224.
157. Achiron R, Barkai G, Katznelson MB, Mashiach S. Fetal lateral
ventricle choroid plexus cysts: the dilemma of amniocentesis. Obstet
Gynecol 1991;78:815-818.
158. Benacerraf BR, Laboda LA. Cyst of the fetal choroid plexus: a
normal variant? Am J Obstet Gynecol 1989;160:319-321.
159. Nadel AS, Bromley BS, Frigoletto Jr FD, et al. Isolated choroid
plexus cysts in the second-trimester fetus: is amniocentesis really
indicated? Radiology 1992;185:545-548.
160. Platt LD, Carlson DE, Medearis AL, Walla CA. Fetal choroid plexus
cysts in the second trimester of pregnancy: a cause for concern. Am
J Obstet Gynecol 1991;164:1652-1655; discussion 1655-1656.
161. Leonardi MR, Wolfe HM, Lanouette JM, et al. The apparently
isolated choroid plexus cyst: importance of minor abnormalities in
predicting the risk for aneuploidy. Fetal Diagn Ther 1998;13:
49-52.
162. Reinsch RC. Choroid plexus cysts: association with trisomy: prospective review of 16,059 patients. Am J Obstet Gynecol 1997;176:
1381-1383.
163. Coco C, Jeanty P. Karyotyping of fetuses with isolated choroid
plexus cysts is not justified in an unselected population. J Ultrasound
Med 2004;23:899-906.
164. Bronsteen R, Lee W, Vettraino IM, et al. Second-trimester sonography and trisomy 18: the significance of isolated choroid plexus
cysts after an examination that includes the fetal hands. J Ultrasound
Med 2004;23:241-245.
165. Bethune M. Time to reconsider our approach to echogenic intracardiac focus and choroid plexus cysts. Aust NZ J Obstet Gynaecol
2008;48:137-141.
166. Ouzounian JG, Ludington C, Chan S. Isolated choroid plexus cyst
or echogenic cardiac focus on prenatal ultrasound: is genetic amniocentesis indicated? Am J Obstet Gynecol 2007;196:595e1-e3; discussion e3.
167. Cheng PJ, Shaw SW, Soong YK. Association of fetal choroid plexus
cysts with trisomy 18 in a population previously screened by nuchal
translucency thickness measurement. J Soc Gynecol Investig
2006;13:280-284.
Trisomy 13 (Patau Syndrome)
168. Hook EB. Rates of 47, + 13 and 46 translocation D/13 Patau syndrome in live births and comparison with rates in fetal deaths and
at amniocentesis. Am J Hum Genet 1980;32:849-858.
169. Redheendran R, Neu RL, Bannerman RM. Long survival in trisomy13-syndrome: 21 cases including prolonged survival in two patients
11 and 19 years old. Am J Med Genet 1981;8:167-172.
170. Benacerraf BR, Frigoletto Jr FD, Greene MF. Abnormal facial features and extremities in human trisomy syndromes: prenatal ultrasound appearance. Radiology 1986;159:243-246.
171. Benacerraf BR, Miller WA, Frigoletto Jr FD. Sonographic detection
of fetuses with trisomies 13 and 18: accuracy and limitations. Am J
Obstet Gynecol 1988;158:404-409.
172. Watson WJ, Miller RC, Wax JR, et al. Sonographic detection of
trisomy 13 in the first and second trimesters of pregnancy. J Ultrasound Med 2007;26:1209-1214.
173. Papp C, Beke A, Ban Z, et al. Prenatal diagnosis of trisomy 13:
analysis of 28 cases. J Ultrasound Med 2006;25:429-435.
174. Lehman CD, Nyberg DA, Winter 3rd TC, et al. Trisomy 13 syndrome: prenatal ultrasound findings in a review of 33 cases. Radiology 1995;194:217-222.
Triploidy
175. Ferguson-Smith MA, Yates JR. Maternal age specific rates for chromosome aberrations and factors influencing them: report of a collaborative European study on 52,965 amniocenteses. Prenat Diagn
1984;4 Spec No:5-44.
1144 PART IV ■ Obstetric Sonography
176. Jacobs PA, Szulman AE, Funkhouser J, et al. Human triploidy:
relationship between parental origin of the additional haploid complement and development of partial hydatidiform mole. Ann Hum
Genet 1982;46:223-231.
177. McFadden DE, Robinson WP. Phenotype of triploid embryos.
J Med Genet 2006;43:609-612.
178. Jauniaux E, Brown R, Rodeck C, Nicolaides KH. Prenatal diagnosis
of triploidy during the second trimester of pregnancy. Obstet
Gynecol 1996;88:983-989.
179. Jauniaux E, Brown R, Snijders RJ, et al. Early prenatal diagnosis of
triploidy. Am J Obstet Gynecol 1997;176:550-554.
180. Rijhsinghani A, Yankowitz J, Strauss RA, et al. Risk of preeclampsia
in second-trimester triploid pregnancies. Obstet Gynecol 1997;90:
884-888.
181. Goldstein DP, Berkowitz RS. Current management of complete and
partial molar pregnancy. J Reprod Med 1994;39:139-146.
Turner (45,X) Syndrome
182. Baena N, De Vigan C, Cariati E, et al. Turner syndrome: evaluation
of prenatal diagnosis in 19 European registries. Am J Med Genet A
2004;129A:16-20.
183. Wax JR, Blakemore KJ, Baser I, Stetten G. Isolated fetal ascites
detected by sonography: an unusual presentation of Turner syndrome. Obstet Gynecol 1992;79:862-863.
184. Azar GB, Snijders RJ, Gosden C, Nicolaides KH. Fetal nuchal cystic
hygromata: associated malformations and chromosomal defects.
Fetal Diagn Ther 1991;6:46-57.
185. Papp C, Beke A, Mezei G, et al. Prenatal diagnosis of Turner syndrome: report on 69 cases. J Ultrasound Med 2006;25:711-717;
quiz 718-720.
CHAPTER 32
Multifetal Pregnancy
Tejas S. Mehta
Chapter Outline
INCIDENCE
Assisted Reproductive Technology
Maternal Age
Race and Geography
Family History
Parity and Body Habitus
ZYGOSITY AND PLACENTATION
SONOGRAPHIC DETERMINATION
OF CHORIONICITY AND
AMNIONICITY
Membrane Thickness
Female Gender
Placenta
Umbilical Cord
Accuracy
MORBIDITY AND MORTALITY
Intrauterine Fetal Demise
Structural Anomalies
Other Screening Tests for Anomalies
Growth Restriction and Discordant
Growth
Premature Delivery
Cervical Incompetence
Placental Abnormalities: Marginal
and Velamentous Insertion
Umbilical Cord Doppler Ultrasound
Multiple gestations have become more common in
the United States and are associated with increased morbidity and mortality compared with singleton births.
The higher the number of fetuses the greater the number
of risks associated with the pregnancy. Fetuses with a
shared placenta have certain risks that fetuses with their
own placenta do not have. When assessing a pregnant
patient, it is important not only to identify if multiple
gestations are present, but also to determine the number
and placentation of the fetuses early in gestation. This
will enable the physician to counsel the patient accurately about potential risks associated with the pregnancy, to screen for these risks appropriately, and to care
for the patient as needed.
COMPLICATIONS
Monochorionic Twins
Twin-Twin Transfusion Syndrome
Twin Embolization Syndrome
Twin Reversed Arterial Perfusion
Sequence
Monoamniotic Twins
Cord Entanglement
Conjoined Twin
SELECTIVE MULTIFETAL
REDUCTION
CONCLUSION
and geography, family history, parity, and body habitus.
Higher education level and socioeconomic status of the
woman have been reported as factors, although these are
thought to be linked to use of ART.
2
RISK FACTORS FOR MULTIFETAL
PREGNANCY
Assisted reproductive therapy
Increased maternal age
Race/geography
Family history
Increased parity
Obesity
INCIDENCE
The number of multiple births in the United States has
risen dramatically over the past 3 decades. Twins account
for 3.2% of all live births
2
In the United States in 2005, twins and higher-
births.
order multiple gestations accounted for 31.1 and 1.8 per
1000 live births, respectively.
factors contributing to this change are the use of assisted
reproductive technology (ART) and delaying childbearing to a later age. Other influential factors include race
1
and 94% of all multiple
3
The two most important
Assisted Reproductive Technology
A common type of ART is in vitro fertilization (IVF).
In the early practice of IVF, multiple embryos were
transferred to obtain a higher rate of achieving and maintaining pregnancy. More recent studies have shown that
transfer of fewer embryos can still yield a successful pregnancy. With as few as two embryos transferred, the rate
of dizygotic twins is as high as 28%.
one embryo is transferred, that embryo can cleave into
monozygotic twins.
6
4,5
Even when only
1145
1146 PART IV ■ Obstetric Sonography
Maternal Age
Dizygotic twinning occurs more frequently in older
women, even without fertility therapy. The incidence of
naturally conceived twins increases fourfold between
ages 15 and 35 years,
is maximum hormonal stimulation and increased rate
of double ovulation.
7
with peak age at 37, when there
8
The trend toward delaying childbearing to a later age, without factoring in use of ART,
accounts for up to 33% of the increase in multiple
2
births.
Race and Geography
Some black populations of Africa have the highest rate
of naturally conceived twins, at 1 in 30, whereas some
Asian populations have much lower rates of less than 1
in 100. The incidence in Caucasians falls between these
two groups at 1 in 80.
twins and higher-order gestations occur in whites than
in Hispanics or blacks.
9,10
In the United States, more
2
Family History
The genetic component for twins can be inherited by
either parent but is thought to be expressed in women.
11
If a woman is a dizygotic twin, the rate of giving birth
to twins is 1 in 58. If the husband is a twin and the
woman is not, the rate of twinning is 1 in 116.
12
Parity and Body Habitus
Increase in parity is associated with increased rate of
twinning, even when controlling for maternal age.
Women who are obese (body mass index [BMI] ≥
7,13
30 kg/
m2) and women who are tall (≥65 inches [162 cm]) are
more likely to have dizygotic twins than women who
are underweight (BMI <
short (<
61 inches [152 cm]).
20 kg/m2) and women who are
14,15
ZYGOSITY AND PLACENTATION
Zygosity refers to the type of conception. If twins arise
from fertilization of two sperm and two ova, they are
dizygotic twins or fraternal twins. In this situation,
there are two blastocysts that form, resulting in two
placentas, and subsequently two chorions, amnions, and
fetuses. Monozygotic twins or “identical” twins result
when there is fertilization of one sperm and one ovum
into one zygote, which then undergoes cleavage to result
in twins. For spontaneous conception, dizygotic twins
are more common than monozygotic twins, at a 70:30
16
The frequency of dizygotic twins compared to
ratio.
monozygotic twins in the setting of ART with multiple
embryo transfers is much higher, at a 95:5 ratio.
Chorionicity refers to type of placentation. In a dizygotic gestation, each zygote forms its own placenta, and
thus each fetus has its own chorion and amnion, a diam-
niotic dichorionic twin gestation.
In a monozygotic gestation, the chorionicity and
amnionicity are determined by when cleavage occurs.
When there is early cleavage of a zygote before blastocyst
formation, which is before day 4 after fertilization, the
result is two blastocysts. The blastocyst implants in the
endometrial cavity and eventually forms the placenta,
chorion, amnion, and fetus. Thus, with early cleavage,
two blastocysts result in two placentas and two fetuses,
each with its own chorion. As the chorion forms before
the amnion, if there are two chorions, there must be two
amnions (diamniotic dichorionic twin gestation).
If late cleavage occurs between days 4 and 8 after
fertilization, the blastocyst has already formed, and thus
there is only one placenta. What cleaves at this point is
the inner cell mass. The amnion has not yet formed, and
thus cleavage during this time results in a gestation with
one chorion and two amnions, a diamniotic monocho-
rionic pregnancy.
If cleavage occurs 8 days after fertilization, at a time
after both the chorion and the amnion have formed,
what cleaves is the embryonic disc; this results in a
monoamniotic (and thus monochorionic) twin pregnancy. A conjoined twin results if there is incomplete
cleavage of the embryonic disc by day 13 (Table 32-1).
All dizygotic twins and one third of monozygotic
twins are dichorionic, resulting in 80% of all natural
twins. Almost two thirds of monozygotic twins are
monochorionic diamniotic. Monoamniotic twins are
rare, representing less than 1% of monozygotic twins
(Fig. 32-1).
17
18
TABLE 32-1. EVENTS INVOLVING PLACENTATION IN A MONOZYGOTIC GESTATION
DAYS AFTER FERTILIZATION UNIT THAT CLEAVES CHORIONICITY AMNIONICITY
Before 4 days Zygote Dichorionic Diamniotic
4-8 days Inner cell mass Monochorionic Diamniotic
8-12 days* Embryonic disc Monochorionic Monoamniotic
*Incomplete cleavage of embryonic disc by day 13 results in conjoined twins.
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