Reproduction · Process
Gestation, the placenta and birth
For nine months one organism lives inside another and takes everything it needs from it — without their blood ever meeting.
Start here
The mother's blood and the foetus's blood never mix.
They are two separate circulations, kept separate for the whole of the pregnancy. And yet the foetus does not breathe, does not eat, and does not use its kidneys — everything it needs arrives, and everything it produces leaves.
So how does oxygen get from one to the other?
It diffuses. In the placenta the two blood supplies are brought within a fraction of a millimetre of each other across an enormous surface, and substances move between them from where there is more to where there is less. No pumping, no mixing, no decision — the same process you met in the alveoli and the small intestine, solving a third version of the same problem.
At the exchange surface · six substances
Which way does it cross?
0 of 6 checked
Pick a substance, commit to a direction, then check it. Every answer comes from the same rule: things move from where there is more of them to where there is less.
Oxygen
The foetus respires like any other organism and cannot use its own lungs — they are full of fluid.
Glucose
The foetus needs glucose for respiration and raw material for building, and does not eat.
Carbon dioxide
Respiration in the foetus produces it continuously, and the foetus cannot breathe it out.
Urea
Breaking down surplus amino acids produces urea, which is toxic if it accumulates.
Antibodies
A newborn’s immune system has never met anything, and takes months to become useful.
Nicotine, alcohol and many drugs
These are small molecules dissolved in the blood, exactly like oxygen and glucose.
Which way does it cross the placenta?
CorrectOther way round
Into the foetus, from the mother’s blood.
The mother’s blood arrives at the placenta freshly oxygenated from her lungs; the foetal blood arrives having just delivered its oxygen to foetal tissue. More on one side, less on the other, so oxygen diffuses across. Foetal haemoglobin also holds oxygen more tightly than the adult kind, which keeps the difference up.
CorrectOther way round
Into the foetus, from the mother’s blood.
This is the far end of the mother’s digestion: her small intestine absorbed the glucose hours earlier and her blood is carrying it. The placenta moves it across into foetal blood, where the concentration is lower because the foetus is continually using it up.
CorrectOther way round
Out to the mother’s blood — and she breathes it out.
Carbon dioxide builds up in foetal blood, so there is more of it there than in the mother’s blood arriving at the placenta. It diffuses across the other way and leaves her body at her alveoli. Two organisms, one set of lungs.
CorrectOther way round
Out to the mother’s blood — and her kidneys remove it.
The foetus has kidneys, and they do make urine, which passes into the amniotic fluid. But the urea itself has to leave the system altogether, and the only route out is across the placenta into the mother’s blood. Her kidneys are doing the work for both.
CorrectOther way round
Into the foetus — but not by diffusion.
Antibodies are large proteins, far too big to slip across an exchange surface built for small molecules. The placenta carries them across deliberately, using energy, and stocks the foetus with a copy of the mother’s immunity that lasts for the first few months of life. It is the one crossing on this list that the placenta chooses to make.
CorrectOther way round
Into the foetus — for the same reason oxygen crosses.
The placenta is not a filter and has no way of telling a useful small molecule from a harmful one. Anything small enough and dissolved in the mother’s blood crosses down its concentration difference, and within minutes the foetus has it too. What follows from that is the whole of the next lesson.
Forty weeks, in five stages
Building the supply line comes first. Everything else depends on it.
Implantation · About five days after fertilisation
The ball of cells embeds in the thickened lining of the uterus. Part of it will become the embryo; another part immediately begins building the structure that will supply it.
The exchange surface is built · Weeks 1–12
The placenta grows into the uterus wall, folding into thousands of finger-like projections until its surface area is enormous, and the umbilical cord connects it to the embryo. Two circulations are brought within a fraction of a millimetre of each other. Nothing that follows is possible without this.
Organs are laid down · Weeks 3–8 · the embryo
Heart, brain, spine, limbs and every other organ are formed while the embryo is smaller than your thumb. This is the shortest stage and the one where anything crossing the placenta has the largest effect.
Growth and maturing · Weeks 9–40 · the foetus
The organs that exist get bigger and finish developing. The lungs are among the last to be ready, which is why being born early is mostly a breathing problem.
Birth · Around 40 weeks
The muscular wall of the uterus begins contracting and the cervix opens. The contractions push the baby out through the vagina, the umbilical cord is cut, and the placenta — no longer needed — is delivered afterwards.
Key fact
The placenta is an exchange surface, built to the same specification as the alveoli and the villi: a very large surface area, a very thin barrier, and a blood supply on both sides to keep the concentration difference up. The two blood supplies come close enough for substances to diffuse across, and never mix.
Think again
“The baby's blood mixes with the mother's blood in the placenta.”
Two circulations, side by side, never joined. If they did mix, two things would go wrong immediately. The first is blood groups: a mother and her baby often have different ones, and mixing incompatible blood makes it clot. The second is pressure — an adult heart pushes blood far harder than a foetal one, and the delicate vessels of the placenta would simply be destroyed. What the placenta does instead is bring the two supplies within a fraction of a millimetre of each other over an enormous folded surface, so that dissolved substances can diffuse across the gap. Everything the foetus receives arrives that way, one molecule at a time, down a concentration difference. The cord is not a pipe carrying the mother's blood; it carries the foetus's own blood out to the exchange surface and back.
“The baby breathes and eats inside the uterus.”
It does neither, and the reason is the same both times: there is nowhere for either to happen. The lungs are full of fluid for the whole pregnancy and no gas exchange takes place in them — the first breath at birth is genuinely the first time they are used, which is why the lungs are among the last organs to be ready and why babies born very early often need help with breathing. Food is the same story in a different organ. Nothing that could be called a meal goes down the umbilical cord: what crosses the placenta is glucose, amino acids and other small molecules already dissolved in blood, which is what the mother's digestive system has spent hours turning food into. The foetus is supplied at the end of somebody else's digestion.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the surface
Why does oxygen move from the mother’s blood into the foetus’s blood at the placenta?
Rung 2 · The one that catches people
A student says the umbilical cord carries the mother’s blood into the baby. What is wrong with that?
Rung 3 · Explain the surface
Explain how the placenta supplies a foetus with oxygen and glucose and removes its carbon dioxide and urea, without the two bloods mixing. Name the three features that make it a good exchange surface.
Rung 4 · Take it somewhere new
Alveoli, villi and the placenta are three different organs in three different systems. Explain what problem all three are solving, what design features they share, and one way the placenta’s problem is harder than the other two.
Key note
Gestation is the time from fertilisation to birth, about 38 weeks in humans — the familiar 40 weeks is counted from the first day of the last period. The placenta is the exchange surface between two separate circulations: oxygen, glucose and other small molecules diffuse in from the mother's blood, carbon dioxide and urea diffuse out into it, and the two never mix. The umbilical cord carries the foetus's own blood to and from the placenta. Organs are laid down in the first eight weeks; the rest is growth. At birth the uterus muscle contracts, the cervix opens, the baby is delivered, and the placenta follows.
Going further
The placenta is not the mother's organ. It grows from the same ball of cells as the embryo, so its cells carry the embryo's chromosomes — half of them from the father — and it is the only organ a human being ever builds, uses and then discards. That raises a question nobody has fully answered. An immune system attacks tissue whose cells it does not recognise; that is why transplanted organs are rejected without drugs to prevent it. Yet here is an organ, and an entire foetus, carrying a set of foreign proteins, growing inside another person's body for nine months, pressed right up against her blood supply, and normally not attacked at all. Several mechanisms are known — the outer layer of the placenta displays unusual surface proteins, and the immune response in the uterus is locally dialled down — but how they add up to nine months of tolerance is still an active research question. The most ordinary event in biology is also one of its stranger unsolved problems.
Before this lesson
- This follows on from The menstrual cycle.
Connects to
At GCSE this becomes
- Exchange surfaces compared quantitatively, the hormones of pregnancy and birth, and the ethics of antenatal screening.
Where to next
Ask Mr Badmus AI
Want to check why the two bloods must stay apart?
If any of this is about you or someone you know, talk to someone you trust — a parent or carer, a teacher, your school nurse, a pharmacist or your GP. Out of school hours: Childline — 0800 1111, free and confidential.
Times are typical values. Pregnancy is dated in weeks from the first day of the last period rather than from fertilisation, so the familiar forty weeks is about two weeks longer than the time the foetus has existed. This lesson covers the biology; anything personal belongs with your school nurse, doctor or midwife, and relationships and consent are covered in RSE and PSHE.
Lesson content © MrBadmusAI.