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Chapter 42 ■ Fetal Measurements: Normal and Abnormal Fetal Growth 1457
FIGURE 42-3. Embryonic heartbeat. Transvaginal
sonogram and M-mode at 6 weeks demonstrate cardiac activity
(calipers) originating from tiny embryo (arrow) adjacent to the
yolk sac.
FIGURE 42-4. Crown-rump length (CRL) measure-
ment. Cursors delineate the length of the fetus from the top of
its head to the bottom of its torso. The yolk sac (arrow) should
not be included in the fetal CRL measurements.
TABLE 42-2. GESTATIONAL DATING BY
ULTRASOUND IN THE FIRST TRIMESTER
SONOGRAPHIC FINDING
Gestational sac, no yolk sac,
embryo, or heartbeat
Gestational sac with yolk sac,
no embryo or heartbeat
Gestational sac with heartbeat
and embryo <
length
Embryo/fetus
length
5 mm in
5 mm in
GESTATIONAL AGE
(weeks)
5
5.5
6
Age based on crown-rump
length (see Table 42-3)
From 6 weeks until the end of the first trimester,
gestational age correlates closely with the crown-rump
length (CRL) of the embryo or fetus.
embryo is used up to 8 to 10 weeks’ gestation, and the
term fetus applies thereafter.
16
The CRL is the length of
14,15
The term
the embryo or fetus from the top of its head to the
bottom of its torso. It is measured as the longest dimension of the embryo, excluding the yolk sac and extremities (Fig. 42-4). The CRL can be used to assign gestational
age accurately up to 11 weeks because minimal biologic
variability occurs during this time (Table 42-3). After
12 to 13 weeks’ gestation, the CRL of the longer, more
developed fetus becomes less reliable. At this later stage,
the CRL is affected by the fetal position, measuring
shorter in a fetus whose spine is flexed and longer in a
fetus whose spine is extended.
The accuracy of gestational age determination by
ultrasound, as measured by the width of the 95% confidence range, is approximately ±0.5 week throughout
the first trimester.
14,15
The sonographic estimation of
gestational age will be within 0.5 week of the actual age
in 95% of cases.
Second and Third Trimesters
Many sonographic parameters have been proposed for
estimating gestational age in the second and third trimesters. These include several fetal measurements: bipa-
rietal diameter (BPD),
abdominal circumference (AC),
18,21-23
(FL),
distance,
measurements: the corrected-BPD
formulas.
length of other long bones,22 and binocular
24
as well as combinations of two or more fetal
20,26
Measurements of structurally abnormal
17,18
head circumference (HC),19
20
femur length
25
and composite age
fetal body parts should not be used in the assignment of
gestational age.
Fetal Head Measurements
Three measurements or parameters involve the fetal
head: BPD, corrected-BPD, and HC. All three measurements are taken from transaxial sonograms of the fetal
1458 PART IV ■ Obstetric Sonography
= ×
( )
[
+
TABLE 42-3. GESTATIONAL AGE
ESTIMATION* BY CROWN-RUMP
LENGTH (CRL)
CRL
(mm)
5 6.0 45 11.1
6 6.2 46 11.2
7 6.4 47 11.3
8 6.6 48 11.4
9 6.8 49 11.4
10 7.0 50 11.5
11 7.2 51 11.6
12 7.4 52 11.7
13 7.5 53 11.8
14 7.7 54 11.8
15 7.8 55 11.9
16 8.0 56 12.0
17 8.1 57 12.1
18 8.3 58 12.2
19 8.4 59 12.2
20 8.5 60 12.3
21 8.7 61 12.4
22 8.8 62 12.4
23 8.9 63 12.5
24 9.0 64 12.6
25 9.1 65 12.7
26 9.3 66 12.7
27 9.4 67 12.8
28 9.5 68 12.9
29 9.6 69 12.9
30 9.7 70 13.0
31 9.8 71 13.1
32 9.9 72 13.2
33 10.0 73 13.2
34 10.1 74 13.3
35 10.2 75 13.4
36 10.3 76 13.4
37 10.4 77 13.5
38 10.5 78 13.5
39 10.6 79 13.6
40 10.7 80 13.7
41 10.8
42 10.8
43 10.9
44 11.0
*Values derived from formula in Robinson HP, Fleming JE. A critical evaluation of
sonar “crown-rump length” measurements. Br J Obstet Gynaecol
1975;82:702-710.
head at the level of the paired thalami and cavum septi
pellucidi
GESTATIONAL
AGE (weeks)
27
(Fig. 42-5). The BPD is measured from the
CRL
(mm)
GESTATIONAL
AGE (weeks)
outer edge of the cranium nearest the transducer to the
inner edge of the cranium farthest from the transducer
(Fig. 42-5). The occipitofrontal diameter (OFD) is
obtained from the same transaxial image as the BPD and
is measured from midskull to midskull along the long
axis of the fetal head (Fig. 42-5). This latter measurement is used in conjunction with the BPD to calculate
the corrected-BPD using the following formula
Corrected-BPD BPD OFD= ×
( )
25
:
1 265.
The rationale for the corrected-BPD is that it repre-
sents the BPD of the standard-shaped head (one with an
FIGURE 42-5. Biparietal diameter (BPD) and
occipitofrontal diameter (OFD) measurements.
Transaxial sonogram of the fetal head at the level of the paired
thalami (arrow), with BPD (calipers 1) and OFD (calipers 2). Note
how the calipers for the BPD are placed from the outer aspect of
the skull to the inner aspect of the skull.
OFD/BPD ratio of 1.265) of the same cross-sectional
25
The same tables or formulas used to determine
area.
gestational age from the BPD are used to determine
gestational age from the corrected-BPD (Table 42-4).
The HC is the length of the outer perimeter of the
cranium, made on the same transaxial image of the fetal
head. It can be measured by using an electronic ellipse
available on most ultrasound scanners
28
(Fig. 42-6 and
Table 42-5). Alternatively, it can be calculated from the
outer-edge-to-outer-edge analogs of the BPD and OFD:
HC Outer-to-outer BPD
1 57.
( )
Outer-to-outer OFD
]
Although the BPD is simpler to measure than the
corrected-BPD or HC, it has the disadvantage of being
the only one of the three measurements that disregards
head shape. This means that two heads of equal widths
but different lengths will have the same BPD, but the
longer head will have a greater corrected-BPD and HC
than the shorter head (Fig. 42-7). The fetus with the
longer head will therefore be assigned a greater gestational age based on the corrected-BPD or HC; however,
both fetuses will be assigned the same gestational age if
the BPD is used as the basis for age assignment.
Femur Length
The length of the diaphysis of the fetal femur is often
used for gestational age prediction.
18,21,22
Careful measurement of the ossified diaphysis of the femur is necessary to obtain an accurate estimate of gestational age by
Chapter 42 ■ Fetal Measurements: Normal and Abnormal Fetal Growth 1459
TABLE 42-4. GESTATIONAL AGE
ESTIMATION* BY BIPARIETAL
DIAMETER (BPD)
BPD OR
BPDc
(mm)
20 13.2 60 24.2
21 13.4 61 24.5
22 13.6 62 24.9
23 13.8 63 25.3
24 14.0 64 25.7
25 14.3 65 26.1
26 14.5 66 26.5
27 14.7 67 26.9
28 14.9 68 27.3
29 15.1 69 27.7
30 15.4 70 28.1
31 15.6 71 28.5
32 15.8 72 29.0
33 16.1 73 29.4
34 16.3 74 29.9
35 16.6 75 30.3
36 16.8 76 30.8
37 17.1 77 31.2
38 17.3 78 31.7
39 17.6 79 32.2
40 17.9 80 32.7
41 18.1 81 33.2
42 18.4 82 33.7
43 18.7 83 34.2
44 19.0 84 34.7
45 19.3 85 35.2
46 19.6 86 35.8
47 19.9 87 36.3
48 20.2 88 36.9
49 20.5 89 37.4
50 20.8 90 38.0
51 21.1 91 38.6
52 21.4 92 39.2
53 21.7 93 39.8
54 22.1 94 40.4
55 22.4 95 41.0
56 22.8 96 41.6
57 23.1
58 23.5
59 23.8
*Values from Doubilet PM, Benson CB. Improved prediction of gestational age in
the late third trimester. J Ultrasound Med 1993;12:647-653.
BPDc, Corrected-BPD.
GESTATIONAL
AGE (weeks)
BPD OR
BPDc
(mm)
≥97
GESTATIONAL
AGE (weeks)
42.0
FL (Fig. 42-8 and Table 42-6). The transducer must be
aligned to the long axis of the diaphysis; this can be
ensured by demonstrating that both the femoral head or
greater trochanter and the femoral condyle are simultaneously in the plane of section. The cursors should be
positioned at the junction of the bone with the cartilage,
and the thin, bright reflection of the cartilaginous epiphysis should not be included in the measurement.
29
Abdominal Circumference
The fetal AC is the length of the outer perimeter of the
fetal abdomen, measured on transverse scan at the level
FIGURE 42-6. Head circumference (HC) measure-
ment. HC measurement (calipers and tracing dots) on transaxial
sonogram of the fetal head at the same level as for the biparietal
diameter measurement. Note how the HC measurement is
obtained from around the bone.
TABLE 42-5. GESTATIONAL AGE
ESTIMATION* BY HEAD
CIRCUMFERENCE (HC)
HC
(mm)
80 13.4 225 24.5
85 13.7 230 25.0
90 14.0 235 25.5
95 14.3 240 26.1
100 14.7 245 26.6
105 15.0 250 27.1
110 15.3 255 27.7
115 15.6 260 28.3
120 16.0 265 28.9
125 16.3 270 29.4
130 16.6 275 30.0
135 17.0 280 30.7
140 17.3 285 31.3
145 17.7 290 31.9
150 18.1 295 32.6
155 18.4 300 33.3
160 18.8 305 33.9
165 19.2 310 34.6
170 19.6 315 35.3
175 20.0 320 36.1
180 20.4 325 36.8
185 20.8 330 37.6
190 21.3 335 38.3
195 21.7 340 39.1
200 22.2 345 39.9
205 22.6 350 40.7
210 23.1 355 41.6
215 23.6 360 42.4
220 24.0
*Values derived from formula in Law RG, MacRae KD. Head circumference as an
index of fetal age. J Ultrasound Med 1982;1:281-288.
GESTATIONAL
AGE (weeks)
HC
(mm)
GESTATIONAL
AGE (weeks)
1460 PART IV ■ Obstetric Sonography
( )
FIGURE 42-7. Effect of head shape on
corrected-BPD and HC. Heads A and B have
equal biparietal diameters (BPD), but A has a smaller
occipitofrontal diameter (OFD) than B. Therefore
the corrected-BPD and head circumference (HC) are
smaller for A than B. Based on BPD, fetuses A and
B would be assigned the same gestational age. Based
on corrected-BPD or HC, however, fetus A would
be assigned a lower gestational age than fetus B.
A B
TABLE 42-6. GESTATIONAL AGE
ESTIMATION* BY FEMUR LENGTH (FL)
FIGURE 42-8. Femur length (FL) measurement.
Electronic calipers measure the ossified diaphysis of the femur.
Note how the bone is imaged close to parallel to the transducer,
and the femur closest to the maternal abdominal wall is
measured.
of the stomach and intrahepatic portion of the umbilical
vein (Fig. 42-9). Alternatively, the AC may be calculated
with equivalent results from two orthogonal abdominal
diameters (AD
transverse, measured on the same image, as follows
, AD2), one anteroposterior and the other
1
AC AD AD= × +
1 57
.
1 2
28,30
:
FL
(mm)
10 13.7 45 24.5
11 13.9 46 24.9
12 14.2 47 25.3
13 14.4 48 25.7
14 14.6 49 26.2
15 14.9 50 26.6
16 15.1 51 27.0
17 15.4 52 27.5
18 15.6 53 28.0
19 15.9 54 28.4
20 16.2 55 28.9
21 16.4 56 29.4
22 16.7 57 29.9
23 17.0 58 30.4
24 17.3 59 30.9
25 17.6 60 31.4
26 17.9 61 31.9
27 18.2 62 32.5
28 18.5 63 33.0
29 18.8 64 33.6
30 19.1 65 34.1
31 19.4 66 34.7
32 19.7 67 35.3
33 20.1 68 35.9
34 20.4 69 36.5
35 20.7 70 37.1
36 21.1 71 37.7
37 21.4 72 38.3
38 21.8 73 39.0
39 22.2 74 39.6
40 22.5 75 40.3
41 22.9 76 40.9
42 23.3 77 41.6
43 23.7
44 24.1
GESTATIONAL
AGE (weeks)
FL
(mm)
≥78
GESTATIONAL
AGE (weeks)
42.0
Composite Formulas and Accuracy
Gestational age can be estimated from measurements of
the head, abdomen, or femur by means of tables or
formulas that present the mean value of each measurement for a given gestational age (see Tables 42-4 to
42-6). Composite age formulas that combine several
fetal measurements can also be used to predict gestational age.
21,26
*Values from Doubilet PM, Benson CB. Improved prediction of gestational age in
the late third trimester. J Ultrasound Med 1993;12:647-653.
The accuracy of gestational age determination
ranges from 1.2 weeks for the HC and corrected-BPD
between 14 and 20 weeks, to 3.5 weeks in the late third
trimester for the FL. As pregnancy progresses, each
parameter becomes less accurate.
18,31,32
The two fetal
Chapter 42 ■ Fetal Measurements: Normal and Abnormal Fetal Growth 1461
head measurements that take head shape into account,
corrected-BPD and HC, are equivalent in accuracy and
more accurate than the BPD throughout gestation. In
the second trimester, corrected-BPD and HC are the
best predictors of gestational age. In the third trimester,
these two head measurements, the FL, and the composite age formulas all predict gestational age with comparable accuracy.
18,32,33
Composite age formulas use two or more measurements in conjunction to estimate gestational age. A
potential disadvantage of using such formulas is that an
abnormal measurement or anomaly might be obscured.
For example, in a fetus with a skeletal dysplasia manifested by shortened long bones and a normal head size,
the gestational age based on the composite formula will
be an underestimation, falling between that predicted by
A
the corrected-BPD and that predicted by the short FL.
As a result, the FL might not appear to be abnormally
small when compared to this improperly calculated gestational age.
Assignment of Gestational Age
The recommended approach to gestational age assignment at the time of the first sonogram is presented in
Table 42-7. In the second and third trimesters, the
choice depends on which measurements are available,
because two or more parameters may be equivalent in
accuracy. In some cases, especially when the initial scan
occurs late in pregnancy, the clinician must decide
whether to use clinical or sonographic criteria to deter-
B
FIGURE 42-9. Abdominal diameter and circumfer-
ence measurements. A and B, Axial views of the fetal
abdomen at the level of the stomach (S) and intrahepatic portion
of the umbilical vein (arrow). On A the transverse (calipers 1) and
anteroposterior (calipers 2) diameters have been measured with
electronic calipers. On B the circumference of the abdomen has
been traced electronically (calipers and tracing dots).
mine the gestational age. As a general rule, we recommend using ultrasound criteria up to 24 weeks of
gestation and the LMP (if clearly recalled) thereafter.
Because fetal measurements become progressively less
accurate predictors of gestational age as pregnancy pro-
34-36
gresses,
the age assigned at the time of the first scan
TABLE 42-7. APPROACH TO GESTATIONAL AGE (GA) ASSIGNMENT BY ULTRASOUND
ON INITIAL SCAN
STAGE OF PREGNANCY BASIS FOR GA TABLES ACCURACY (weeks)*
Early (5-6 weeks) Sonographic milestones 42-1
Mid- to late (6-13 weeks) CRL 42-2
If OFD measurable BPDc or HC 42-3, 42-4
If OFD not measurable BPD or FL 42-3, 42-5
If OFD measurable BPDc, HC, or FL 42-3, 42-4, 42-5
If OFD not measurable FL 42-5
First Trimester
Second Trimester
Third Trimester
±0.5
±0.5
±
1.2 (14-20 wk)
±1.9 (20-26 wk)
±
1.4 (14-20 wk)
±2.1-2.5 (20-26 wk)
±±
3.1-3.4 (26-32 wk)
±±3.5-3.8 (32-42 wk)
±±
3.1 (26-32 wk)
±±3.5 (36-42 wk)
34
*Two standard deviations (2 SD), or 95% confidence interval (CI).
CRL, Crown-rump length; OFD, occipitofrontal diameter; BPD, biparietal diameter; BPDc, corrected-BPD; HC, head circumference; FL, femur length.
1462 PART IV ■ Obstetric Sonography
should not be changed thereafter. The age at any time
later in pregnancy should be based on the initial sono-
graphic study, calculated by taking the gestational age
assigned at the time of the first scan and adding the
number of weeks that have elapsed since that scan. On
subsequent examinations, standard fetal measurements
(BPD, OFD, AC, and FL) should be obtained and
should be compared to the normal standards for the
gestational age, based on the initial sonogram, to determine whether the fetus is appropriate in size.
WEIGHT ESTIMATION
AND ASSESSMENT
Estimation of Fetal Weight
Before the availability of ultrasound, manual examination of the maternal abdomen was the only approach
that could be used to estimate fetal size. The physical
examination, however, provides only a general approximation of fetal weight because the palpated dimensions
of the uterus are affected by several factors other than
fetal size, including amniotic fluid volume, placental
bulk, presence of fibroids, and maternal obesity.
Sonographic measurements of fetal body parts provide
a direct way of assessing fetal size. Numerous formulas
have been published for estimating fetal weight from one
or more of these fetal body measurements: head (BPD
or HC), abdomen (AD or AC), and femur (FL).
Other measurements, such as thigh circumference,
have been used as well.
46
Formulas that estimate fetal
37-46
weight using three-dimensional (3-D) sonography
and 3-D magnetic resonance imaging (MRI) have also
been published.
50,51
The accuracy of a weight prediction formula is
determined by assessing how well the formula works
in a group of fetuses scanned close to delivery. An
important measure of a formula’s performance is its
95% confidence range. If the 95% confidence range is
±18%, for example, the estimated weight will fall within
18% of the actual weight in 95% of cases, and the error
will be greater than 18% in only 5% of cases. The narrower the confidence range, the more reliable is the
formula.
Many published studies provide information that
allows one to estimate this measure of a formula’s
accuracy
52,53
(Table 42-8). The following points are
noteworthy:
• The accuracy of weight prediction formulas
improves as the number of measured body parts
increases up to three, achieving greatest accuracy
when measurements of the head, abdomen, and
femur are used. There is no apparent improvement
by adding the thigh circumference as a fourth
measurement,
54
and no proven benefit from using
3-D sonography or MRI.
• Even when based on measurements of the head,
abdomen, and femur, sonographic weight prediction
has a rather wide 95% confidence range of at least
±15%. Based on the abdomen and either the head
or femur, the range is at least ±16%-18%. Precision
is considerably worse when only the abdomen
is used.
47-49
TABLE 42-8. ACCURACY OF FETAL WEIGHT PREDICTION FORMULAS
BODY PART(S) INCLUDED
IN FORMULA FORMULA* 95% CONFIDENCE RANGE (%)†
Abdomen Campbell and Wilkin
Head and abdomen Warsof et al.
Abdomen and femur Hadlock et al.
Head, abdomen, and femur Hadlock et al.
Head, abdomen, femur, and thigh Vintzileos et al.
*Study in which formula was developed.
†Computed as two standard deviations (2 SD) of the relative error, as reported in the study(ies) referenced, unless otherwise indicated.
‡Based on the fraction of cases in which the estimated weight falls within 10% of the actual weight.
Higginbottom et al.
Hadlock et al.
Vintzileos et al.
Shepard et al.
Thurneau et al.
42
Jordaan
Hadlock et al.
Hadlock et al.
45
Birnholz
Vintzileos et al.
Hadlock et al.
Hadlock et al.
Vintzileos et al.
37
38
43
46
39
40
41
43
44
46
43
44
43
44
46
46
±17.1-23.8
±23.8
±22.2
±22.8
±17.4-21.2
±18.2-18.3
±19.8
±25.8
±18.2
±18.2
±17.753‡
±21.2
±16.4
±16.0
±15.0-15.4
±14.8-15.0
±17.6
±15.6-17.8
43,52
43
43
46
39,43,57
40,52
43
43
43
44
46
43
44
43
44
46
46
Chapter 42 ■ Fetal Measurements: Normal and Abnormal Fetal Growth 1463
• A number of factors have been studied to determine
their effect on accuracy of weight prediction.
Accuracy appears to be worse in fetuses that weigh
under 1000 grams than in larger fetuses.
rest of the birth weight range, however, accuracy is
fairly constant.
43,44,52,55
Weight prediction is less
53
Over the
accurate in diabetic than in nondiabetic mothers. In
diabetic mothers, formulas that use measurements
of the head, abdomen, and femur have a 95%
confidence range of ±24%,
±15% in the general population.
oligohydramnios or polyhydramnios has no impact
on accuracy.
41,53,57
Scan quality may have an effect
56
wider than the range of
43,44
The presence of
on accuracy. Studies have shown a trend toward
greater accuracy in scans that were rated “good”
compared with those rated “poor” based on ability
to visualize anatomic landmarks.
53,58
Recommended Approach
An attempt should be made to image all three key fetal
anatomic regions—head, abdomen, and femur—at the
appropriate anatomic levels (Table 42-9). If measure-
ments of all three structures can be obtained, Formula 1
in Table 42-9 should be used to estimate fetal weight.
This formula should be used with the corrected-BPD
when the OFD is available, and with the BPD itself if
not. An alternative approach, equally accurate but more
cumbersome, would be to use Formula 1 when the OFD
is unavailable, and a formula based on HC, AC, and FL
when the OFD is available. If the abdomen and only the
head or the femur can be appropriately imaged, Formula
2 or 3 should be used. If the abdomen cannot be measured, or both the head and femur cannot be measured,
then a weight estimate should not be calculated. Using
the approach outlined in Table 42-9, an accuracy of
±15%-18% can be achieved for weight estimation.
Weight Assessment in Relation
to Gestational Age
When an ultrasound is performed in the third trimester,
best estimates of gestational age and fetal weight should
be established. The gestational age may be based on a
prior ultrasound, clinical dating criteria, or current measurements; fetal weight is always calculated from current
measurements. The two values should be cross-assessed
to determine whether the fetus is appropriate in size for
dates. This can be accomplished by using a table that
provides norms of values for fetal weight as a function
of gestational age (Table 42-10), several of which appear
in the literature.
59-64
As an example, suppose that an obstetric sonogram
reveals the best estimated gestational age is 34 weeks.
According to Table 42-10, a weight of 2146 grams (g)
TABLE 42-9. APPROACH TO FETAL
WEIGHT ESTIMATION
BODY PARTS IMAGED
OFD measurable Formula 1, using corrected-BPD
OFD not measurable Formula 1
OFD measurable Formula 2, using corrected-BPD
OFD not measurable Formula 2
— Formula 3
( )
Log EFW AC FL BPD
10 2
( )
Log EFW AC AC
10
(
Log EFW AC FL AC FL
10
*Formulas from Hadlock FP, Harrist RB, Sharman RS, et al. Estimation of fetal
weight with the use of head, body, and femur measurements: a prospective study.
Am J Obstet Gynecol 1985;151:333-337.
EFW, Estimated fetal weight, in grams (g); BPD, biparietal diameter (cm); AC,
abdominal circumference (cm); FL, femur length (cm); OFD, occipitofrontal
diameter (cm).
Head, Abdomen, and Femur
Head and Abdomen
Abdomen and Femur
1 4787 0 003343 0 001837
>
= − × + +
. . .
AC
0 0458 0
. ..158 FL
1 1134 0 05845 0 000604
>
= + − −
. . .
0 007365 0 0
)
1 3598 0 051 0 1844 0 0037
= + + − ×. . . .
BPD
. . 00595 0 1694BPD AC BPD× + .
FORMULA USED FOR
WEIGHT ESTIMATE
in place of BPD
in place of BPD
Formula 1*
+
Formula 2*
2
+
Formula 3*
2
TABLE 42-10. FETAL WEIGHT
PERCENTILES IN THE THIRD TRIMESTER
GESTATIONAL
AGE (weeks)
25 490 660 889
26 568 760 1016
27 660 875 1160
28 765 1005 1322
29 884 1153 1504
30 1020 1319 1706
31 1171 1502 1928
32 1338 1702 2167
33 1519 1918 2421
34 1714 2146 2687
35 1919 2383 2959
36 2129 2622 3230
37 2340 2859 3493
38 2544 3083 3736
39 2735 3288 3952
40 2904 3462 4127
41 3042 3597 4254
42 3142 3685 4322
43 3195 3717 4324
From Doubilet PM, Benson CB, Nadel AS, Ringer SA. Improved birth weight
table for neonates developed from gestations dated by early ultrasonography. J
Ultrasound Med 1997;16:241-249.
Weight Percentiles (Grams)
10th 50th 90th
1464 PART IV ■ Obstetric Sonography
corresponds to the 50th percentile, and weights of
1714 g and 2687 g correspond to the 10th and 90th
percentiles, respectively. A weight between the 10th and
90th percentiles is generally considered to be “appropriate for gestational age.” When the estimated weight falls
outside this range, the diagnosis of a small-for-gestational-age or large-for-gestational-age fetus is suggested.
When fetal weight is estimated on a third-trimester
sonogram and a weight percentile is determined, correct
interpretation of that percentile should take into account
how weight percentile tables are derived. Such tables are,
of necessity, derived from birth weights of neonates,
versus estimated weights of fetuses, because only neonatal weights are known. For example, the mean and
standard deviation of weight at 27 weeks’ gestation is
determined from data on birth weights of babies born at
27 weeks’ gestation. It is important to note that several
studies have shown that small fetuses have an increased
likelihood of early delivery, so neonates born at 27 weeks’
gestation are, on average, smaller than fetuses remaining
in utero at that gestational age.
65-67
It follows that more
than 50% of 27-week fetuses will have an estimated
weight above the 50th percentile, and fewer than 10%
will fall below the 10th percentile.
The weight gain between two ultrasound examinations can be estimated as the difference between the two
estimated weights. Adequacy of weight gain can be
assessed by comparing this difference to established
normal fetal growth rate as a function of gestational age.
Brenner’s data indicate that median fetal weight gain per
week increases progressively until 36 weeks of gestation,
reaching a maximum rate of 220 grams per week.
59,60
After 36 weeks, the rate of weight gain steadily decreases
in the normal fetus. The longer the time between scans,
the more accurate is the sonographic estimate of interval
weight gain. When two scans are performed within 1
week of each other, weight gain cannot be determined
reliably, so there is little or no value in computing an
estimated weight at the time of the second scan.
When several examinations have been performed,
fetal growth can be depicted graphically by means of a
trend plot, or growth curve. One form of growth curve
plots the estimated fetal weight versus gestational age,
with the curve for the fetus being examined superimposed on lines depicting the 1st, 10th, 50th, 90th, and
99th percentiles (Fig. 42-10, A). An alternative mode of
display plots the estimated weight percentile versus
gestational age (Fig. 42-10, B). In this latter format, the
graph for a normally growing fetus will be a horizontal
line, indicating maintenance of a particular weight percentile throughout gestation. A downsloping line indicates a subnormal growth rate, and an upsloping line
indicates accelerated growth.
Calculation of weight percentiles and plotting of
growth curves is most easily accomplished by computer,
using an obstetric ultrasound software package that performs these tasks.
68-70
Alternatively, similar results can be
A
B
FIGURE 42-10. Fetal growth curves. A, Estimated fetal
weight plotted against gestational age, superimposed on 1st, 10th,
50th, 90th, and 99th percentile curves. The fetus depicted here
has a normal growth pattern, with estimated fetal weights between
the 50th and 90th percentile over four sonograms. B, Estimated
fetal weight (EFW) percentile against gestational age.
achieved by means of a calculator and manual plotting
of data.
FETAL GROWTH ABNORMALITIES
The Large Fetus
The large-for-gestational-age (LGA) neonate (or fetus)
is defined as one whose weight is above the 90th percentile for gestational age.
entity, is most often defined on the basis of a weight
above 4000 g; other weight cutoffs (4100 g, 4500 g) are
sometimes used.
62,64-66
with different frequencies and are associated with different morbidities and mortalities in diabetic mothers than
in the general population. Therefore these two patient
populations are considered separately.
General Population
About 10% of all infants have birth weights above the
90th percentile for gestational age and are considered
59,71-73
Macrosomia, a related
These growth disturbances occur
Chapter 42 ■ Fetal Measurements: Normal and Abnormal Fetal Growth 1465
LGA infants. Of all newborns, 8% to 10% have birth
weights over 4000 g and thus are classified as “macrosomic,” and 2% weigh over 4500 g.
72,74-76
Risk factors
for LGA and macrosomia include maternal obesity,
diabetes, history of a previous LGA infant, prolonged
pregnancy (>40 weeks), excess pregnancy weight gain,
multiparity, and advanced maternal age.
71,72,74,77-79
Large fetuses have an increased incidence of perinatal
morbidity and mortality, in large part because of obstetric complications. Shoulder dystocia, fractures, and facial
and brachial plexus palsies occur more frequently as a
result of traumatic delivery.
77,80,81
The incidence of perinatal asphyxia, meconium aspiration, neonatal hypoglycemia, and other metabolic complications is significantly
increased in these pregnancies.
71,74,77
The most straightforward approach to diagnosing
LGA and macrosomia is to use the estimated fetal weight
computed from sonographic measurements. An estimated weight above the 90th percentile for gestational
age suggests LGA, and a weight estimate above 4000 g
suggests macrosomia. Although weight estimation is less
accurate in large than in average-sized fetuses,
52,82-84
this
approach has been demonstrated to be moderately good
for diagnosing LGA and macrosomia. It has a positive
predictive value (PPV) of up to 51% for LGA and 67%
for macrosomia. Other proposed sonographic parameters have lower sensitivity or lower PPV than the estimated fetal weight
52,71,82,85-90
(Table 42-11).
Diabetic Mothers
Fetuses of insulin-dependent and gestational diabetic
mothers are exposed to high levels of glucose throughout
pregnancy and, as a result, produce excess insulin. This
leads to overgrowth of the fetal trunk and abdominal
organs, while the head and brain grow at a normal
72,74
rate.
Therefore, these fetuses tend to have different
body proportions than fetuses of nondiabetic mothers.
Sonographic measurements of fetuses of diabetic mothers
demonstrate accelerated growth of the fetal thorax
and abdomen beginning between 28 and 32 weeks’
gestation.
72,73,91
An LGA weight occurs in 25% to 42% and macrosomia in 10% to 50% of infants of diabetic mothers
(IDMs).
72,73,92
As many as 12% of IDMs weigh more
than 4500 g at birth. Perinatal complications are more
frequent in macrosomic fetuses of diabetic mothers than
in those of nondiabetic mothers.
76,80,81,93,94
Shoulder dystocia, for example, occurs in 31% of macrosomic fetuses
of diabetic mothers and only 3% to 10% of macrosomic
fetuses of nondiabetic mothers.
77,80
Many sonographic parameters, involving a variety of
measurements, formulas, and ratios, have been proposed
for diagnosing LGA and macrosomia in the fetus of the
diabetic mother
85,95-97
(Table 42-12). As a group, these
have higher sensitivities and PPVs than sonographic criteria in the general population, in part because of the
higher prevalence of large fetuses in diabetic mothers.
As in the general population, the most straightforward
approach to diagnosing LGA and macrosomia in the
fetuses of diabetic mothers is by means of the sonographically estimated fetal weight.
56,85,95,98,99
A fetus
whose estimated weight falls above the 90th percentile
for gestational age has a 74% likelihood of being LGA,
versus 19% if the estimated weight lies below the 90th
percentile.
95
A weight estimate above 4000 g is associated with a 77% chance of macrosomia, and one above
4500 g with an 86% chance. The chance of macrosomia
TABLE 42-11. SONOGRAPHIC CRITERIA FOR LARGE-FOR-GESTATIONAL AGE (LGA) AND
MACROSOMIA IN THE GENERAL POPULATION: PERFORMANCE CHARACTERISTICS
(%) Predictive Values (%)*
SENSITIVITY SPECIFICITY POSITIVE NEGATIVE
Criteria to Predict LGA*
Criteria to Predict Macrosomia
112
assuming an LGA prevalence rate of 10%.
Elevated AD-BPD
Low FL/AC
Elevated AFV
Elevated ponderal index
High EFW
Elevated growth score
Elevated AFV, high EFW
Elevated FL
Elevated AC
High EFW
Elevated BPD
From Doubilet PM, Benson CB. Fetal growth disturbances. Semin Roentgenol 1990;25:309-316.
*Predictive values for criteria for LGA computed using Bayes’ theorem,
AD, Abdominal diameter; BPD, biparietal diameter; FL/AC, femur length/abdominal circumference ratio; AFV, amniotic fluid volume; EFW, estimated fetal weight; FL,
femur length; AC, abdominal circumference.
71,86
87,88
71,88
89
89
53,84,89
89
86
71,86
71
88
46 79 19 93
24-75 44-93 13-26 92-94
12-17 92-98 19-35 91
13-15 85-98 13-36 91-94
20-74 93-96 6-51 88-94
14 91 10 90
11 99 54 99
24 96 52 88
53 94 63 89
11-65 89-96 38-67 83-91
29 98 71 92
1466 PART IV ■ Obstetric Sonography
TABLE 42-12. SONOGRAPHIC CRITERIA FOR LARGE-FOR-GESTATIONAL AGE (LGA) AND
MACROSOMIA IN DIABETIC MOTHERS: PERFORMANCE CHARACTERISTICS
(%) Predictive Values (%)
SENSITIVITY SPECIFICITY POSITIVE NEGATIVE
Criteria to Predict LGA*
Criteria to Predict Macrosomia
112
assuming an LGA prevalence rate of 10%.
Intrauterine growth restriction has been categorized as
symmetrical or asymmetrical. Fetuses with symmetrical
IUGR are proportionately reduced in size, whereas
in asymmetrical
portionately small in relation to the head and limbs.
IUGR the fetal abdomen is dispro-
95
73,96
97
56
95
95
73,93,95
93
96
73
95
92
50 80 64 70
83 60 71 75
78 78 74 81
13 86 75 57
71-88 81-85 56-78 81-96
84 85 79 89
58-79 75-80 68-83 75-76
72 71 89 89
84 78 41 96
48-64 60-74 36-42 80-83
87 72 61 92
48 95 77 84
Elevated HC
Elevated AC/BPD
High EFW
Elevated BPD
Elevated AC
Elevated AC growth
Low FL/AC
Elevated AC, high EFW
Elevated AC
Low FL/AC
Elevated TD-BPD
High EFW
From Doubilet PM, Benson CB. Fetal growth disturbances. Semin Roentgenol 1990;25:309-16.
*Predicted values for criteria for LGA computed using Bayes’ theorem,
HC, Head circumference; AC/BPD, abdominal circumference/biparietal diameter ratio; EFW, estimated fetal weight; FL/AC, femur length/abdominal circumference ratio;
TD, thoracic diameter.
is only 16% when the weight estimate is less than
4000 g.56 It follows that if vaginal delivery is believed to
be contraindicated for the macrosomic fetuses of diabetic
mothers, the estimated fetal weight should be considered
when selecting the route of delivery.
There is considerable overlap between these two groups,
Intrauterine Growth Restriction
Intrauterine growth restriction (IUGR) is a fetal growth
disorder most often defined on the basis of a weight
below the 10th percentile for gestational age.
100-104
This
disorder is sometimes termed small for gestational age
(SGA); however, it should be recognized that some
authors use the term SGA to describe fetuses measuring
less than the 10th percentile that are constitutionally
small, and differentiate these from fetuses with abnormal
growth restriction.
Most cases of growth restriction are caused by placental
insufficiency, either primary or secondary to a maternal
etiology such as hypertension, collagen vascular disease,
poor nutrition, or substance abuse. IUGR may also result
from a chromosomal anomaly (e.g., trisomy 18) or intrauterine infection (e.g., cytomegalovirus).
101,104-106
In many cases, the specific cause of IUGR cannot be
determined prenatally. As a group, regardless of the etiology, growth-restricted fetuses have a poor prognosis,
with increased perinatal morbidity and mortality. Their
mortality rate is four to eight times that of non-IUGR
105,106
fetuses.
One half of surviving growth-restricted
infants have serious short-term or long-term morbidity,
including meconium aspiration, pneumonia, and metabolic disorders.
104,105,107,108
however, so this categorization is probably not useful
clinically.
ventional and Doppler ultrasound, have been proposed
for antenatal diagnosis of IUGR.
useful for diagnosis, a criterion must detect a substantial
fraction of cases of growth restriction (i.e., its sensitivity
must be high), and a positive result must be associated
with a high likelihood of IUGR (i.e., its PPV must be
high). Similarly, to be valuable for excluding IUGR, a
criterion must have high specificity and high negative
predictive value (NPV).
graphic criteria for IUGR are presented in Table 42-13,
listed in order of increasing PPV.
the HC/AC ratio, with a PPV of 62%. Even when based
on this criterion, IUGR cannot be diagnosed with confidence because more than one third (38%) of fetuses
with an abnormal HC/AC ratio will not be growth
restricted. Other parameters have even lower PPV,
with seven of the nine parameters listed having PPV
under 50%.
the mid-1980s. Early studies evaluated the use of Doppler
to diagnose IUGR. In particular, Doppler was used
to assess blood flow in the fetoplacental or uteroplacental circulations, both of which are essential for fetal
109
Numerous sonographic parameters, using both con-
110,111
To be clinically
112
The performance characteristics of conventional sono-
110
The best criterion is
Doppler became readily available for clinical use in
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