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Placenta

The placenta is a foetal organ of pregnancy, which is responsible for providing nutrition and oxygen to the foetus as well as excretory functions. Ultrasound is the first-line modality for imaging the placenta, but MRI may be indicated if an abnormality is suspected. 

Embryology

Placenta is formed by foetal and maternal components 1:

  • 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 umbilical artery), protruding into intervillous spaces as chorionic villi

Gross anatomy

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

The term placenta weighs ~470 g and measures ~22 cm in diameter with a thickness of 2.0-2.5 cm 3Placental 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 changing morphology of placental substance with increasing gestation, maturity grading of placenta is conveniently done.

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

Variant anatomy

Variant morphologies may be frequently encountered 1:

Radiographic features

Ultrasound

Ultrasound is the first-line modality in imaging the placenta due its wide availability and its use of non-ionising 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

MRI is usually a problem-solving modality for placental assessment. The suspicion of abnormality is usually raised either by prior ultrasound or prior obstetric history. In all routine pregnancies, MRI is not indicated owing to the tissue heating effect of MRI 2.

For further evaluation, see: Placental evaluation with MRI.

History and etymology

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".


Related articles

Abdominal and pelvic anatomy
Ultrasound - obstetric

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