Anatomy chapters
Fetal membranes, umbilical cord and placenta
How the placenta forms and works, what the amnion and chorion are, what the umbilical cord contains, and the abnormalities exams ask about.
- The placenta has a fetal part (chorion frondosum) and a maternal part (decidua basalis).
- The umbilical cord has two arteries and one vein in Wharton's jelly.
- Exams test amniotic fluid problems, abnormal placental attachment and twinning.
On this page
Overview
The placenta is the organ of exchange between mother and fetus. It also makes hormones.
The fetus is wrapped in two membranes, the amnion (inner) and chorion (outer). It is joined to the placenta by the umbilical cord.
The chorion and decidua
The chorion is formed by the trophoblast and extraembryonic mesoderm. Early on, villi cover its whole surface.
- Chorion frondosum: villi on the side facing the uterine wall grow and branch. This becomes the fetal part of the placenta.
- Chorion laeve: villi on the opposite side disappear, leaving a smooth membrane.
After implantation, the endometrium is called the decidua:
| Part | Position | Fate |
|---|---|---|
| Decidua basalis | Between the embryo and the uterine muscle | Maternal part of the placenta |
| Decidua capsularis | Over the embryo, facing the uterine cavity | Thins and fuses with the decidua parietalis |
| Decidua parietalis | Lines the rest of the uterus | Fuses with the capsularis by about month 4 |
Placenta = chorion frondosum (fetal) + decidua basalis (maternal).
Structure of the placenta
At term the placenta is a disc about 15 to 25 cm across and weighs about 500 g.
- The fetal surface is smooth, covered by amnion, with the cord attached near the centre.
- The maternal surface is divided into 15 to 20 cotyledons by grooves.
Maternal blood fills the intervillous spaces. Fetal blood runs in capillaries inside the villi. The two do not mix.
The placental barrier
Early in pregnancy the barrier has four layers: syncytiotrophoblast, cytotrophoblast, connective tissue of the villus and fetal capillary endothelium. Later, the cytotrophoblast largely disappears and the barrier becomes much thinner.
Functions of the placenta
- Exchange: oxygen, carbon dioxide, nutrients, water and waste products.
- Antibodies: maternal IgG crosses and protects the baby for the first months.
- Hormones: hCG, progesterone, oestrogens and human placental lactogen. From about the fourth month the placenta makes enough progesterone to keep the pregnancy going.
The placenta is not a perfect barrier. Many drugs, alcohol, and some viruses (rubella, cytomegalovirus) and the parasite of toxoplasmosis cross it.
IgM does not cross the placenta. IgM in a newborn's blood points to infection in the baby itself.
The amnion and amniotic fluid
The amnion is a thin membrane that surrounds the fetus. It grows until it fills the chorionic cavity and fuses with the chorion.
Amniotic fluid cushions the fetus, lets it move, keeps temperature steady and helps the lungs develop. The fetus swallows it and adds urine to it.
At term the volume is roughly 800 to 1,000 mL.
| Problem | Meaning | Classic causes |
|---|---|---|
| Polyhydramnios | Too much fluid | Fetus cannot swallow: oesophageal atresia, anencephaly; maternal diabetes |
| Oligohydramnios | Too little fluid | Fetus makes no urine: renal agenesis; leaking membranes |
Renal agenesis causes oligohydramnios, which squeezes the fetus and stops the lungs growing (Potter sequence: small lungs, flattened face, limb deformities).
The umbilical cord
The cord forms from the connecting stalk. At birth it is usually about 50 to 60 cm long.
It contains:
- Two umbilical arteries: carry deoxygenated blood from the fetus to the placenta.
- One umbilical vein: carries oxygenated blood to the fetus.
- Wharton's jelly: mucous connective tissue that protects the vessels.
- Remnants of the allantois and the vitelline duct.
In the fetus the umbilical vein carries the best-oxygenated blood, the opposite of the usual vein rule.
A single umbilical artery is linked with other birth defects, especially of the heart and kidneys, so the baby should be examined carefully.
Twins
- Dizygotic (fraternal) twins: two oocytes fertilised by two sperm. They always have two amnions, two chorions and two placentas (which may fuse).
- Monozygotic (identical) twins: one embryo splits. The time of the split decides the membranes.
| Time of split | Membranes |
|---|---|
| Two-cell stage | Two chorions, two amnions |
| Inner cell mass stage (most common) | One chorion, two amnions |
| Bilaminar disc stage | One chorion, one amnion |
| Incomplete split later | Conjoined twins |
Clinical relevance
Placenta praevia
How it happens: the placenta lies over or near the internal os.
What you see: painless bright red bleeding in late pregnancy.
Abnormal attachment
How it happens: the decidua is missing or thin, often over a previous caesarean scar.
What you see: the placenta does not separate after birth, causing severe bleeding. Accreta: villi attach to the myometrium. Increta: villi enter the myometrium. Percreta: villi pass through it, sometimes into the bladder.
Cord attachment variants
Battledore placenta: the cord attaches at the edge of the placenta. Velamentous insertion: the vessels run through the membranes before reaching the placenta and can tear.
Expect: the parts forming the placenta, the contents of the cord, which umbilical vessel carries oxygenated blood, causes of polyhydramnios and oligohydramnios, the meaning of a single umbilical artery, and how the timing of the split decides twin membranes.
Quick revision
- Placenta: chorion frondosum + decidua basalis.
- Term placenta about 500 g with 15–20 cotyledons; blood of mother and fetus does not mix.
- IgG crosses the placenta; IgM does not.
- Cord: two arteries (deoxygenated), one vein (oxygenated), Wharton's jelly.
- Polyhydramnios: oesophageal atresia, anencephaly. Oligohydramnios: renal agenesis.
- Most monozygotic twins: one chorion, two amnions.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
What forms the maternal part of the placenta?
The decidua basalis.
How many vessels are in the umbilical cord?
Three: two umbilical arteries and one umbilical vein.
What causes polyhydramnios?
Anything that stops the fetus swallowing, such as oesophageal atresia or anencephaly, and maternal diabetes.
What is placenta accreta?
Abnormal attachment of the placental villi directly to the myometrium, so the placenta does not separate after delivery.