Week 1 to 2: implantation
About six to seven days after fertilisation, the blastocyst attaches to the uterine lining. Its outer layer, the trophoblast, invades the lining and begins to differentiate. Some of these cells fuse into a syncytium that erodes maternal blood vessels — this invasion is what creates the blood supply the pregnancy will depend on, and how well it happens in these first weeks is closely linked to later risks of pre-eclampsia and growth restriction.
Weeks 3 to 5: villi and circulation
Finger-like projections called chorionic villi form and branch into the maternal tissue. Fetal blood vessels grow into them. Maternal blood pools in spaces around the villi, so maternal and fetal blood come within a few cell layers of each other without ever mixing — the exchange surface of the whole pregnancy. The yolk sac, visible on early ultrasound, nourishes the embryo during this handover.
Weeks 6 to 10: remodelling the arteries
Trophoblast cells migrate into the walls of the maternal spiral arteries and convert them from narrow, muscular vessels into wide, low-resistance channels. This is the single most important step in placental development. Incomplete remodelling produces a high-resistance circulation, and is the underlying mechanism of early-onset pre-eclampsia and fetal growth restriction. It is largely complete by about 20 weeks.
Weeks 10 to 12: the hormonal handover
Until now the corpus luteum in the ovary has maintained the pregnancy with progesterone. Between weeks 7 and 10 the placenta takes over, and by week 12 it is the dominant source of progesterone, oestrogen, hCG and human placental lactogen — the hormone that drives the insulin resistance behind gestational diabetes.
Weeks 12 to 20: maturation
The villi branch further, the barrier between maternal and fetal blood thins, and surface area multiplies. By 20 weeks the structure is essentially final. From then to term the placenta grows in size and surface area, reaching roughly 500 g and 22 cm across, while the exchange barrier continues to thin.
What the placenta does
- Transfers oxygen and nutrients in, and carbon dioxide and waste out.
- Produces the hormones that maintain pregnancy and prepare the breasts for feeding.
- Transfers maternal antibodies from about 28 weeks, giving the newborn passive immunity — the reason vaccination in the third trimester protects the baby.
- Filters selectively: it blocks some substances and freely admits others, including alcohol, nicotine and many medicines.
Placental problems worth knowing about
- Placenta praevia — the placenta covers or sits near the cervix. Often resolves as the uterus grows; if it persists, delivery is by caesarean.
- Placenta accreta spectrum — abnormally deep attachment, more likely after previous caesareans. Needs planned specialist delivery.
- Abruption — premature separation, presenting with pain and bleeding. An emergency.
- Placental insufficiency — poor function causing growth restriction, detected by growth scans and Doppler studies.
- Low-lying or fundal position — noted on scan and usually of no consequence on its own.
Report any bleeding, constant abdominal pain, or reduced fetal movement straight away. And attend your growth scans — placental function is the thing they are really assessing.