4.36.1\ Scanning Method

After locating the placenta, the probe is moved sagittally to find the cervical long axis, then locate the inner os of cervix, and observe the relationship between the placental margin and the inner os of cervix.

4.36.2\ Section Structures

The filled bladder and the lower segment of the uterus in the back of the bladder, as well as the cervix, placenta, and placental margin, are shown.

4.36.3\ Measuring Method

Measure the distance between the placental margin and the inner os of the cervix.

4.36.4\ Clinical Significance

(1) When the distance between the placental margin and the inner os of cervix is less than 30 mm, regular follow-up is suggested. The placental

lower margin may shift upward along with the formation of lower uterine segment (Fig. 13.49). Placenta previa could be diagnosed after 28 weeks of gestation; (2) It is defined as a complete (central) placenta previa when the lower placental margin crosses one side of cervical inner os and arrives at the opposite side (Fig. 13.50); (3) It is defined as marginal placenta previa if the lower placental margin only arrives at the edge of cervical inner os, without crossing the cervical inner os or arriving at the opposite side; (4) It is defined as partial placenta previa if the lower margin of placenta goes beyond one side of cervical inner os without arriving the opposite side, which is difficult to be diagnosed and not easily to be differentiated from the marginal type.

Amniotic Fluid

Amniotic fluid is anechoic in early stage; the light spots increase in third trimester because of falling off of fetal skin cells and fetal vernixes.