The placenta in late pregnancy
- Lesion
In later pregnancy the placenta can fail from the mother's side (poor blood supply) or the baby's side (cord problems), and twins add their own risks.
What it is
As fetal demands rise in the second and third trimesters, placental disease becomes a major cause of fetal growth restriction and stillbirth. Maternal vascular malperfusion means too little maternal blood reaches the intervillous space, usually because the spiral arteries were not remodelled; it causes a small placenta with infarcts and accelerated villous maturation. Fetal vascular malperfusion means obstructed flow in the fetal circulation, often from the umbilical cord, and leaves thrombi and avascular villi. In twins, the type of placenta decides the risks, and monochorionic twins share vessels that can cause twin–twin transfusion.
Structure
- Infarct: a firm, well-demarcated area of villi that died from loss of maternal flow; small peripheral infarcts are common at term, large or central ones are significant.
- Decidual arteriopathy: spiral arteries with fibrinoid necrosis and foamy macrophages (acute atherosis) and persistent muscle in their walls.
- Fetal vascular malperfusion: thrombi in fetal stem vessels and groups of villi without capillaries, their stroma hyalinised.
- Twin placentas: the dividing membrane has four layers (amnion, chorion, chorion, amnion) when dichorionic and two layers (amnion, amnion) when monochorionic.
Step by step
- Maternal side: shallow trophoblast invasion leaves narrow, muscular spiral arteries.
- Flow into the intervillous space is low and pulsatile; villi infarct and the placenta stays small.
- Fetal side: a cord knot or compression slows flow in the umbilical vein; stasis forms thrombi in fetal vessels.
- Downstream villi lose their capillaries and stop exchanging oxygen and nutrients.
- Either way the fetus grows poorly and, if severe or sudden, dies.
Twin placentas: what the membranes reveal
| Type | When the embryo split (monozygotic) | Zygosity | Main risk |
|---|---|---|---|
| Dichorionic diamniotic | Within the first few days, before the chorion forms, or two zygotes | Dizygotic or monozygotic | Lowest of the twin types |
| Monochorionic diamniotic | After the chorion forms, before the amnion | Monozygotic | Twin–twin transfusion |
| Monochorionic monoamniotic | After the amnion forms | Monozygotic | Cord entanglement; later splitting gives conjoined twins |
Clinical notes
- Placental examination after a stillbirth or a growth-restricted baby often explains the cause and the risk in a future pregnancy.
- Maternal vascular malperfusion is the placental counterpart of preeclampsia and chronic hypertension.
- Severe fetal vascular malperfusion is associated with neonatal encephalopathy and cerebral palsy.
Test yourself
Think of the answer, then tap the question to see it.
What does a monochorionic placenta tell you about twins?
They are monozygotic (identical) and share placental vessels, which allows twin–twin transfusion.
What is maternal vascular malperfusion?
Too little maternal blood flow into the intervillous space, usually from unremodelled spiral arteries, giving a small placenta with infarcts and accelerated villous maturation.
What do avascular villi indicate?
Fetal vascular malperfusion: obstruction upstream in the fetal circulation, often from an umbilical cord problem.
Elsewhere in DurgiPedia
- Twins: dizygotic and monozygotic, chorionicity, twin–twin transfusion
- Placenta: structure, circulation and functions
- Placenta
- Fetal membranes: amnion, chorion, yolk sac and allantois
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Placental Abruption (StatPearls (NCBI Bookshelf), 2026); Preeclampsia (StatPearls (NCBI Bookshelf), 2026).
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Representative model, for educational purposes only.