Placenta and umbilical cord
Placenta, fetal membranes, umbilical cord and twinning
The placenta is built from the chorion (fetal) and the decidua basalis (maternal), with maternal blood bathing fetal villi in the intervillous space. Its stages of villus formation, the membranes and cord, its abnormal forms and the timing of twin splitting explain a large share of CPSP obstetric anatomy questions, from vasa praevia to twin-twin transfusion.
Trophoblast and villi
- The trophoblast splits into an inner cytotrophoblast (dividing, Langhans cells) and an outer syncytiotrophoblast, which never divides; it gains nuclei by fusion of cytotrophoblast cells, and old nuclei cluster as syncytial knots.
- The syncytium makes hCG (keeps the corpus luteum alive until the placenta takes over steroid production at about 8–10 weeks), hPL, progesterone and oestrogens.
| Villus | Core | Timing |
|---|---|---|
| Primary | Cytotrophoblast covered by syncytium | End of week 2 to start of week 3 |
| Secondary | Extraembryonic mesoderm | Week 3 |
| Tertiary | Mesoderm with fetal capillaries | End of week 3; joins fetal circulation in week 4 |
- Hofbauer cells are fetal macrophages in the villous core (can carry CMV).
- The barrier thins with gestation: the cytotrophoblast becomes patchy, so at term it is mainly syncytium plus fetal endothelium. The placenta is haemochorial: maternal endothelium is never part of the barrier.
Maternal side
- Extravillous cytotrophoblast invades the spiral arteries, replacing their muscle with fibrinoid, so they become wide, low-resistance channels. Shallow invasion underlies pre-eclampsia and growth restriction.
- Spiral arteries open into the intervillous space; maternal blood bathes the villi and drains through endometrial veins. Abruption bleeds from this maternal side.
- Decidual cells are endometrial stromal cells enlarged by progesterone, full of glycogen and lipid.
| Decidua | Position | Fate |
|---|---|---|
| Basalis | Under the conceptus | Maternal part of placenta (basal plate, septa) |
| Capsularis | Over the conceptus | Fuses with parietalis by about the 4th month, obliterating the uterine cavity |
| Parietalis | Rest of the uterus | Part of the membranes |
- Villi facing the decidua basalis grow (chorion frondosum, the fetal part of the placenta); those under the poorly supplied capsularis degenerate (chorion laeve).
- Absent decidua basalis lets villi attach to or invade myometrium: accreta (attached), increta (into), percreta (through the serosa, even into the bladder). Risk: previous caesarean scar with praevia.
The term placenta and its variants
- Disc of about 500 g; maternal surface with 15–20 cotyledons separated by incomplete septa from the decidua basalis; fetal surface smooth, covered by amnion, with the cord usually central.
| Variant | Feature | Risk |
|---|---|---|
| Succenturiate lobe | Accessory lobe joined by membrane vessels | Retained lobe: PPH, infection |
| Circumvallate | Chorionic plate smaller than basal plate, folded membrane ring | Antepartum bleeding, PPROM |
| Velamentous insertion | Cord inserts into membranes; vessels lack Wharton's jelly | Over the os: vasa praevia |
- Vasa praevia: bleeding at membrane rupture with sudden fetal bradycardia while the mother is stable; the blood lost is fetal.
- Transfer: oxygen by simple diffusion (umbilical vein PO2 about 30 mmHg); glucose by facilitated diffusion (GLUT1); IgG by FcRn transcytosis, mostly in the third trimester; insulin does not cross.
- Confined placental mosaicism: CVS samples trophoblast (trophectoderm), the fetus comes from the inner cell mass, so results can differ.
Membranes and amniotic fluid
- Amnion: avascular, covers the fetal surface and the cord. Extraembryonic coelom (chorionic cavity) lies between amnion and chorion, holds the yolk sac, and is obliterated by 12–16 weeks.
- On transvaginal scan the secondary yolk sac (a 3 mm ring) is the first sign confirming an intrauterine pregnancy.
- Amniotic bands: early amnion rupture gives ring constrictions, amputations and asymmetric clefts.
- Fluid: early a transudate, later mainly fetal urine plus lung fluid; removed by swallowing; about 800 mL at term. Oesophageal atresia gives polyhydramnios with no stomach bubble; renal agenesis or urethral valves give oligohydramnios and lung hypoplasia. L/S ratio of 2 or more indicates mature surfactant.
- In chorioamnionitis membrane neutrophils are maternal; funisitis neutrophils are fetal.
Umbilical cord
- Two arteries (from the internal iliac arteries, carrying deoxygenated blood; later medial umbilical ligaments and superior vesical arteries) and one vein (the left persists; oxygenated; later ligamentum teres).
- The vein reaches the left portal vein; the ductus venosus shunts blood past the liver to the IVC (later ligamentum venosum).
- Wharton's jelly: mucous connective tissue from extraembryonic mesoderm, rich in hyaluronic acid, protects the vessels.
- Single umbilical artery (about 1%): look for renal and cardiac anomalies.
Twinning
- Dizygotic (two ova): always dichorionic diamniotic; placentas may fuse. Monozygotic twins depend on timing of the split:
| Split after fertilisation | Type |
|---|---|
| 0–3 days (morula) | DCDA |
| 4–8 days (blastocyst) | MCDA, commonest monozygotic type |
| 8–12 days | MCMA (cord entanglement) |
| After day 13 | Conjoined |
- First-trimester scan at 11–14 weeks: lambda (twin-peak) sign, villi between two chorions, means dichorionic; T sign, a thin membrane of two amnions only, means monochorionic.
- Shared placental anastomoses in monochorionic twins cause twin-twin transfusion (recipient polyhydramnios, donor oligohydramnios, stuck twin) and TRAP (artery-to-artery flow to an acardiac twin; the pump twin fails).
Remember The core defines the villus: trophoblast, then mesoderm, then capillaries.
Exam trap Decidua basalis is the maternal part of the placenta; chorion frondosum is the fetal part.
Exam trap Dichorionic twins can be monozygotic; only monochorionic twins share anastomoses and risk twin-twin transfusion.
Test yourself on this topic
5 of the 51 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
After preterm labour with prolonged rupture of membranes, placental histology shows neutrophils throughout the chorion and amnion and also infiltrating the walls of the umbilical vessels (funisitis). Where do the neutrophils in the umbilical vessel walls come from?
Answer: The fetal circulation
Ascending infection causes acute chorioamnionitis. Neutrophils in the membranes come from the mother, migrating from decidual vessels and the intervillous space, and represent the maternal inflammatory response. Neutrophils in the walls of the umbilical and chorionic plate vessels migrate out of fetal blood and represent the fetal inflammatory response. Funisitis is linked with a higher risk of neonatal sepsis and brain injury.
Chorioamnionitis
- Ascending infection
- Membrane neutrophils: maternal
- Vessel wall neutrophils: fetal
- Funisitis: fetal response
Remember Inflamed cord vessels mean the fetus is reacting.
Exam trap Funisitis signals fetal involvement, not just maternal infection.
Why the other options are wrong
- The maternal intervillous blood Supplies the subchorionic neutrophils of the maternal response.
- The maternal decidual vessels Supply the neutrophils in the membranes.
- The amniotic epithelium Epithelium does not produce neutrophils.
- Bone marrow of the mother Maternal neutrophils do not enter fetal vessel walls.
After a normal delivery, the midwife examines the rough, dark red maternal surface of the placenta. Which structure forms the maternal component of the placenta?
Answer: Decidua basalis
The placenta has a fetal part, the chorion frondosum (villous chorion), and a maternal part, the decidua basalis, which is the decidua lying deep to the implanted conceptus. The decidua basalis forms the basal plate on the maternal surface. The decidua capsularis covers the conceptus on the uterine cavity side, and the decidua parietalis lines the rest of the uterus; neither forms part of the placenta.
Two parts of the placenta
- Fetal: chorion frondosum
- Maternal: decidua basalis
- Decidua basalis forms the basal plate
- Capsularis and parietalis: not placental
Remember Basalis is the base of the placenta.
Exam trap Decidua capsularis sounds placental but forms part of the membranes.
Why the other options are wrong
- Decidua capsularis Covers the conceptus on the cavity side; thins and fuses with parietalis.
- Decidua parietalis Lines the rest of the uterine cavity.
- Chorion frondosum The fetal component.
- Chorion laeve Smooth chorion of the membranes; not placental.
Examining a cut umbilical cord, a student sees three vessels embedded in a pale, gelatinous material that protects them from compression and kinking. What is this material and its origin?
Answer: Mucous connective tissue from mesoderm
Wharton jelly is mucous (mucoid) connective tissue derived from the extraembryonic mesoderm of the connecting stalk. It consists of scattered fibroblast-like cells in a ground substance rich in hyaluronic acid. It cushions the umbilical vessels and resists compression and kinking. Where the cord vessels lack Wharton jelly, as in velamentous insertion, they are vulnerable to compression and rupture.
Wharton jelly
- Mucous connective tissue
- From extraembryonic mesoderm
- Rich in hyaluronic acid
- Protects the vessels
Remember The jelly cushions the cord vessels.
Exam trap Vessels running outside the jelly, in the membranes, are unprotected.
Why the other options are wrong
- Fibrin deposited from maternal blood Fibrin is not the normal cord matrix.
- Gelatinous remains of the yolk sac The yolk sac stalk regresses; it does not form the jelly.
- Hyaline cartilage of the stalk There is no cartilage in the cord.
- Amniotic epithelium thickened by fluid The amnion only covers the cord surface.
After delivery, the umbilical cord is found to insert into the membranes 6 cm from the edge of the placenta, with its vessels running unprotected through the membranes to the disc. What is this abnormality?
Answer: Velamentous cord insertion
In velamentous insertion the cord inserts into the chorion laeve rather than the placental disc, so the cord vessels run between amnion and chorion without the protection of Wharton jelly. These vessels can be compressed or torn, and if they cross the internal os they form vasa praevia, which can bleed catastrophically from the fetal circulation when the membranes rupture. In battledore insertion the cord attaches at the placental edge.
Velamentous insertion
- Cord inserts into membranes
- Vessels without Wharton jelly
- Risk of compression and rupture
- Over the os: vasa praevia
Remember Naked vessels in the membranes are fragile.
Exam trap Bleeding from torn velamentous vessels is fetal blood, so fetal distress is rapid.
Why the other options are wrong
- Battledore (marginal) insertion The cord inserts at the edge of the disc itself.
- Succenturiate lobe An extra lobe joined by membrane vessels; the cord is normal.
- Circumvallate placenta Folded membrane ring on the fetal surface.
- Single umbilical artery Concerns vessel number, not insertion.
A curettage specimen from a very early miscarriage at the start of the third week shows finger-like projections, each with a core of cytotrophoblast covered by syncytiotrophoblast and no mesoderm. What type of villi are these?
Answer: Primary villi
Villi develop in three stages. Primary villi appear at the start of the third week as cores of cytotrophoblast covered by syncytiotrophoblast. When extraembryonic mesoderm grows into the core, they become secondary villi. When blood vessels and capillaries form in that mesoderm, by the end of the third week, they are tertiary (definitive) villi. Hydropic villi are swollen villi seen in molar pregnancy.
Stages of villi
- Primary: cytotrophoblast core
- Secondary: mesoderm core
- Tertiary: capillaries in mesoderm
- All covered by syncytiotrophoblast
Remember Cells, then mesoderm, then vessels.
Exam trap The syncytial covering is present at every stage; the core defines the type.
Why the other options are wrong
- Secondary villi Contain a mesoderm core.
- Tertiary villi Contain fetal capillaries in the mesoderm.
- Stem (anchoring) villi A functional type of mature villus, not a developmental stage.
- Hydropic villi Swollen, avascular villi of molar pregnancy.
Where this sits in the CPSP syllabus
- Obstetrics & Gynaecology, Paper 2: Embryology, Placenta, fetal membranes, umbilical cord and twinning
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