Placenta

Last revised by Craig Hacking on 5 Mar 2024

The placenta is a fetal organ of pregnancy, responsible for providing nutrition and oxygen to the fetus as well as excretory functions. 

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Placenta is formed from fetal and maternal components 2:

  • maternal component: decidua placentalis is the inner portion of the placenta, which is formed by trophoblastic invasion of endometrium
  • fetal component: chorion frondosum is formed by an arterial plexus (branches of the umbilical artery), protruding into intervillous spaces as chorionic villi

Typically, the placenta is discoid in shape. The placenta normally lies along the anterior or posterior wall of the uterus and may extend to the lateral wall with increasing gestational age 1.

The placenta at term weighs ~470 g and measures ~22 cm in diameter with a thickness of 2.0-2.5 cm 3. Placental thickness is usually directly proportional to gestational age, to the extent that it can often predict the gestation weeks (e.g. 21 mm thickness at 21 weeks gestation).

Due to the changing morphology of placental substance with increasing gestation, maturity grading of the placenta is conveniently done.

The umbilical cord typically inserts at the center of the placental bulk.

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Variant morphologies may be frequently encountered 1:

Ultrasound is the first-line modality in imaging the placenta due its wide availability and its use of non-ionizing radiation. The placenta appears as a uniformly echogenic (intermediate echogenicity) structure along uterine wall, with a deep hypoechoic band separating it from normal uterine myometrium. This retroplacental hypoechoic band is vital to rule out implantation disorders and its normal appearance should not be confused with retroplacental hematoma 1. There may also be numerous anechoic areas, representing venous lakes, within the placenta itself 4. 

MRI is usually a problem-solving modality for placental assessment. Suspicion for an abnormality is usually raised either by prior ultrasound or obstetric history. MRI is not routinely indicated due to the tissue heating effect of MRI 5.

For further evaluation, see: placental evaluation with MRI.

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Placenta is derived from the Greek word, plakuos, meaning "flat cake" 1 and this reflects its typical appearance. The placenta was noted by Aristotle and Galen, but the term originates with Fallopius who called it the "placenta uterina".

In cases of complicated pregnancy, early/late fetal demise, placental infections and fetal anomalies, post partum placental assessment is necessary 6.

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