• The placenta is formed embryologically from the outer trophoblast cells of the blastocyst which further differentiate into...

  • The inner proliferative cytotrophoblast 

  • The outer-layer syncytiotrophoblast 

    • Erodes into maternal tissues 

    • Secretes human chorionic gonadotropin (hCG) to maintain the pregnancy, as the progesterone production from the corpus luteum declines during the first trimester. 

• The blastocyst implants within the endometrial canal, usually within the mid to upper half of the uterine cavity.

• Placental tissue is typically visible at 10 weeks gestation at transabdominal or transvaginal US. 

• Intervillous blood flow is seen at Doppler US as early as 12–13 weeks gestation, and the placenta is well formed by 14–15 weeks gestation.

NORMAL US ANATOMY

• A normal placenta is homogeneously hyperechoic opposed to myometrium with a thin (1-2 mm) intervening hypoechoic / anechoic retroplacental clear zone containing the retroplacental complex (RPC).

  • RPC consists of the decidua basalis, maternal vessels, and myometrium.

• The placenta continues to increase in size in the 2nd trimester and may contain venous lakes w/ smooth contours and slow internal flow at Doppler imaging.

• The placenta becomes increasingly vascular in the 3rd trimester and can develop calcs, reflecting the expected placental aging.

• At term, the placenta measures 12-20 cm in diameter and 2-4 cm in thickness.

NORMAL MRI ANATOMY

• Placenta is discoid in morphology w/ smooth tapered margins and homogeneous moderately hyperintense T2WI SI relative to myometrium. 

  • Studies show that >85% retained homo SI from 19 to 23 weeks gestation with 90% demonstrating mild lobulation and heterogeneity which increases w/ age. This may reflect age related infarction and calcification, findings that are seen in a portion of normal placentas at term.

• Thin T2 hypointnse septa surrounding placental cotylendons can often be seen coursing through the normal placenta.

• The adjacent myometrium at the uteroplacental interface is best seen on T2WI and SSFP images and demonstrates a trilaminar appearance:

  • T2 hypointense inner layer

  • T2 intermediate SI middle layer

  • T2 hypointense outer serosal layer

• Trilaminar appearance is often lost in alte gestation w/ a preserved single T2 hypointense line reflecting a combination of the inner and outer layers and compression of the intervening vascular layer.

• Myometrial contractions are often seen in gravid patients which appear as focal low T2SI thickening of the myometrium and distortion of the inner myometrial contour.

• Contractions may be seen anywhere in the uterus, including at the uteroplacental interface and should not be misinterpreted as pathologic. Consecutive sequences often show resolution of this finding.