Introduction
The placenta is the organ of exchange between mother and fetus. At term it is a disc about 20 cm across, 2–3 cm thick and 500 g, about one sixth of the fetal weight.
Structure
- Fetal side (chorionic plate): shiny, covered by amnion, with the cord insertion and branching chorionic vessels.
- Maternal side (basal plate): 15–20 cotyledons separated by septa.
- Villi covered by syncytiotrophoblast float in the intervillous space, which is filled with maternal blood from 100–150 spiral arteries. Maternal and fetal blood never mix (haemochorial placenta).
Trophoblast invasion remodels the spiral arteries into wide, low-resistance vessels. When this fails, the result is preeclampsia and fetal growth restriction.
Functions
Placental hormones
Umbilical cord and membranes
- Cord: 50–60 cm, two arteries and one vein in Wharton's jelly. The arteries carry deoxygenated blood to the placenta. A single umbilical artery is linked with fetal anomalies.
- Abnormal insertions: marginal (battledore) or velamentous (vessels run through the membranes, risk of vasa praevia).
- Membranes: amnion (inner) and chorion (outer).