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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

The placenta normally lies along the anterior or posterior wall of uterus and may extend to lateral wall with increasing gestational age 1. The term placenta weighs ~470g and measures ~22cm in diameter with a thickness of 2.0-2.5cm 3Placental thickness is usually directly proportional to gestation age, to the extent that it can often predict the gestation weeks (e.g. 21 mm thickness at 21 weeks gestation).

Typically, the placenta is discoid in shape. However, variant morphologies may be frequently encountered 1:

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.

Etymology

Placenta is derived from the Greek word, plakuos, meaning flat cake 1 and this reflects its typical appearance. 

Radiological features

Ultrasound

Ultrasound is the first-line modality in imaging the placenta due its wide availabilty and use of non-ionising radation. 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 the modality of choice to assess the placenta. However, the suspicion of abnormality is either raised by prior ultrasound or prior obstetric history. In all routine pregnancies, MRI is not indicated owing to the tissue heating effect of MRI 2. Similar to ultrasound, MRI shows placenta as an isointense structure along the uterine wall with a deeper, hypointense band separating it from the myometrium.


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