The Placenta: How a Baby Is Fed Before Birth

For nine months one temporary organ does the job of a baby’s lungs, gut and kidneys all at once — the placenta, the only organ ever built by two people. It is fundamentally an exchange surface: the mother’s blood and the baby’s blood flow astonishingly close but never mix, separated by a barrier just a few micrometres thick. Across it, oxygen and food pass in to the baby and waste passes back out to the mother. Press play and watch a single chorionic villus trade with a pool of the mother’s blood.

Try this: start on Exchange and watch oxygen ride the umbilical vein to the baby, then switch to What crosses and see alcohol slip straight through the “barrier” into the baby’s blood.

The Science in Plain Language

One organ, built by two people

The placenta is the only organ a human body ever builds jointly with another person. Its outer layer grows from the fetus (from the same fertilized egg), while it burrows into and is fed by the mother’s uterus. By term it is a dark-red disc about 22 cm across weighing roughly 500 grams — and it is essentially a giant exchange surface. The fetal side sends out thousands of branching, finger-like chorionic villi that dip into open pools of the mother’s blood (the intervillous space). Unfolded, the villous surface covers something like 12–14 square metres — about half a badminton court — all packed into a disc you can hold in two hands. The single most important fact is the one the animation shows: the two bloodstreams run within a few micrometres of each other but never actually mix. Everything trades across a barrier.

Breathing for a baby that can’t breathe

A fetus lives underwater, in effect, with lungs that are collapsed and full of fluid. So the placenta becomes its lungs. Oxygen dissolved in the mother’s blood diffuses across the villous membrane into the fetal capillaries and is carried up the umbilical vein to the baby. Carbon dioxide goes the other way — from baby, down the two umbilical arteries, across the barrier, into the mother’s blood, where she breathes it out. (It is a common surprise that in the cord the vein carries the oxygen-rich blood and the arteries carry the oxygen-poor blood — the opposite of the rest of the body, because “artery” just means “away from the heart.”) The same one-way trade handles the kidney’s job: urea and other wastes cross to the mother to be filtered and urinated out. That is why a fetus needs no working lungs and no working kidneys yet.

Fetal haemoglobin: the tighter grip

Here is the beautiful trick that makes it all work. Oxygen only crosses the placenta because the baby’s blood pulls it harder than the mother’s does. Fetal red cells are packed with fetal haemoglobin (HbF), built from two alpha and two gamma chains (α₂γ₂) instead of the adult’s α₂β₂. HbF binds oxygen more tightly: its P50 — the oxygen pressure at which the haemoglobin is half-loaded — is about 19 mmHg, versus roughly 26–27 mmHg for an adult. That “left-shifted” curve means that even in the low-oxygen environment of the placenta (the blood reaching the baby carries a modest partial pressure of oxygen), the fetal cells still fill up to a high saturation of around 75–85%. After birth the body switches the gamma gene off and the beta gene on; by about six months of age the baby’s blood is nearly all adult haemoglobin. This switch is also why inherited disorders of adult haemoglobin, like sickle-cell disease and beta-thalassemia, usually stay silent for the first months of life and only appear once HbF fades — a genuinely useful clue for parents and doctors.

Feeding: glucose, amino acids, and why “the baby takes what it needs” is only half true

Food crosses in three different ways, and the differences matter clinically. Glucose moves by facilitated diffusion through GLUT1 transporters — it flows downhill, from the higher level in the mother to the lower level in the baby. That means the fetus does not get to choose: if the mother’s blood sugar runs high, more glucose floods across, which is exactly the problem in gestational diabetes (below). Amino acids, the building blocks of protein, are different — they are pumped uphill by active transporters, so the baby’s blood can hold higher amino-acid levels than the mother’s. Fats and fatty acids cross more slowly with the help of binding proteins, and key nutrients like iron, calcium and vitamin B12 are actively concentrated on the fetal side. So the old saying that “the baby takes what it needs first” is partly true for minerals and protein — but for sugar and for toxins, the baby simply gets whatever the mother’s blood is carrying.

The hormone factory

The placenta is also one of the busiest endocrine glands the body ever makes. Its first signature hormone is human chorionic gonadotropin (hCG) — the molecule a home pregnancy test detects in urine. In early pregnancy hCG roughly doubles every two to three days, peaking around 8–11 weeks, and its job is to keep the ovary’s corpus luteum producing progesterone so the pregnancy isn’t shed. Around 7–9 weeks the placenta takes over progesterone and estrogen production itself (the “luteoplacental shift”), and hCG then falls back for the rest of pregnancy. The placenta also makes hormones — notably human placental lactogen (hPL) and placental growth hormone — that deliberately make the mother’s tissues resist insulin, so more glucose stays in her blood and flows to the baby. When that insulin resistance is more than the mother’s pancreas can compensate for, blood sugar climbs and the result is gestational diabetes, which affects roughly 6–9% of U.S. pregnancies (CDC). It is usually a placental-hormone problem, not a personal failing — and it typically resolves once the placenta is delivered.

Borrowed immunity: the antibody handoff

A newborn’s own immune system is barely switched on, so the placenta lends it the mother’s. Of the five antibody classes, only IgG is small and correctly shaped to be actively carried across the placenta — it’s grabbed by a shuttle called the neonatal Fc receptor (FcRn) and handed to the fetal side, mostly in the last trimester. The baby is born with a starter set of the mother’s antibodies that protects it for roughly the first three to six months of life, until its own immunity ramps up. This is the entire logic behind vaccinating pregnant women against whooping cough (Tdap) and influenza: the mother makes IgG, the placenta ferries it across, and the newborn arrives already partly protected against illnesses that are most dangerous in the first weeks. (Breast milk then adds a different antibody, IgA, that coats the baby’s gut — a second, separate layer of borrowed defence.)

A leaky gatekeeper — the myth worth correcting

The most important thing to unlearn is the comforting idea that “the placenta is a barrier that filters out anything harmful.” It is not. It is a partial barrier: it blocks many large molecules and most bacteria, but plenty of small molecules pass straight through. Alcohol crosses freely and fast — within an hour the baby’s blood-alcohol level is essentially the same as the mother’s, and because the fetus clears it slowly, alcohol is the leading preventable cause of birth defects (fetal alcohol spectrum disorders). There is no known safe amount or safe trimester, which is why the guidance is simply none. Nicotine crosses and constricts the placental vessels, cutting the baby’s oxygen. Many medications cross, which is why drug labels carry pregnancy warnings. And a specific set of infections can cross — remembered by the sketch “TORCH”: Toxoplasmosis, Rubella, Cytomegalovirus, Herpes, plus syphilis and Zika. The barrier is real, but it is a filter with holes, not a wall.

The immune paradox: why the mother doesn’t reject the baby

A fetus carries the father’s genes as well as the mother’s, which makes it, immunologically, half-foreign tissue — the kind of thing the body normally attacks and rejects, the way it would a transplanted organ. Yet a healthy pregnancy is tolerated for nine months. A big part of the answer sits right at the barrier the animation shows. The placental cells that face the mother’s blood (the syncytiotrophoblast) mostly hide their identity: they don’t display the usual class I MHC markers that would flag them as foreign, and instead show an unusual molecule, HLA-G, that actively calms the mother’s immune cells. The uterine lining is also stocked with specialised uterine natural-killer cells and regulatory T cells that, rather than attacking, help remodel the mother’s spiral arteries so more blood can pour into the intervillous space. When this delicate tolerance is disturbed, it can contribute to conditions like pre-eclampsia and recurrent pregnancy loss. It is one of biology’s most elegant truces: an organ that is half-stranger, welcomed instead of rejected.

Birth, the first breath, and the afterbirth

At delivery the placenta’s job ends abruptly. The baby’s first breath inflates the lungs, oxygen floods in, the pressure in the lung blood vessels drops, and two fetal shortcuts — the foramen ovale (a flap between the heart’s upper chambers) and the ductus arteriosus (a bypass around the lungs) — close over the next hours to days. In a single breath the circulation switches from “lungs off, placenta on” to “lungs on, placenta off.” The placenta then peels away from the uterine wall and is delivered as the afterbirth, usually within about 5 to 30 minutes of the baby — the “third stage” of labour. One honest note against a popular trend: eating the placenta afterward (placentophagy / placenta encapsulation) has no proven health benefit in careful studies, and the U.S. CDC documented a 2017 case in which a newborn developed a group B Streptococcus infection linked to the mother’s contaminated placenta capsules. The organ that fed the baby for nine months has done its work; there is no evidence that consuming it helps.

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Connections

• All Interactive Visualizations
• Hemoglobin & the Oxygen Curve
• The Menstrual Cycle
• Breathing & Gas Exchange
• Gestational Diabetes