Reproduction · System
Lifestyle and the developing foetus
The placenta is an exchange surface, not a filter. What crosses it is decided by the molecule — not by whether the foetus would be better off without it.
Start here
The placenta cannot tell the difference between oxygen and alcohol.
It is a surface built for one job: letting small dissolved molecules move from where there is more of them to where there is less. It has no sorting mechanism, no sense of what is wanted, and no way of stopping something simply because it is harmful.
Which of these reaches the foetus within minutes of entering the mother's blood?
All four. Every one of them is a small molecule that dissolves in blood, and that is the only qualification the crossing requires. Within a few minutes the concentration in the foetus's blood is close to the concentration in the mother's. The interesting question is therefore not does it get through — it is what does it do when it arrives, and does it matter more at some weeks than others.
Six substances · one rule
Does it cross, and what does it reach?
0 of 6 checked
Commit to an answer before you check. By the third one you should be able to predict the rest — the rule turns out to be about the molecule, not about the substance's reputation.
Alcohol
A small molecule that dissolves in both water and fat — which is what lets it into every tissue in an adult too.
Carbon monoxide, from smoke
Produced whenever tobacco burns — and breathed in by anyone in the room, not only the smoker.
Caffeine
In coffee, tea, cola and energy drinks. A stimulant, and the most widely used drug in the world.
Rubella virus
A mild illness in a child — a few days of rash and fever — and a serious one for an embryo.
Prescribed medicines
Someone may need treatment for epilepsy, diabetes, asthma, an infection or a mental health condition while pregnant.
Insulin
The hormone that controls blood glucose, and the treatment for diabetes.
Does it reach the foetus?
CorrectNot this time
Yes — freely, and within minutes.
The foetus ends up with roughly the same concentration in its blood as the mother has in hers, but a much less developed liver to break it down, so it stays there longer. Its clearest effects are on the developing brain and nervous system, and on growth. UK advice is that no amount is known to be safe, which is a statement about missing knowledge as much as about the alcohol.
When it matters most
The whole pregnancy. The brain and nervous system are being built and rebuilt from the first weeks to the last, so there is no window in which alcohol has nothing to reach.
CorrectNot this time
Yes — and it works by taking up space.
Carbon monoxide binds to haemoglobin far more tightly than oxygen does, so every molecule of it in the blood is a seat oxygen cannot use. Less oxygen reaches the placenta, less crosses it, and a foetus short of oxygen grows more slowly — which is why smoking during pregnancy lowers birth weight measurably. Nicotine crosses too, and narrows blood vessels, which reduces the supply again.
When it matters most
Mostly the growth half of the pregnancy, because this is a supply problem rather than a building problem — though the supply matters throughout.
CorrectNot this time
Yes — and this one is about dose.
Caffeine crosses easily and the foetus breaks it down slowly. Unlike alcohol, the advice here is a limit rather than a ban: UK guidance is to stay under about 200 mg a day, roughly two mugs of instant coffee. It is a useful case, because it shows that crosses the placenta and harmful at any amount are two different statements, and the second one has to be established separately for every substance.
When it matters most
No sharply defined window. What the evidence associates with high intake is reduced growth, which can happen at any stage.
CorrectNot this time
Yes — but by a completely different route.
A virus is far too large to diffuse anywhere. It crosses by infecting the placenta’s own cells and being made again on the other side, cell by cell. Catching rubella early in pregnancy can damage the developing eyes, ears and heart, which is why the MMR vaccination given in childhood is as much a protection for future pregnancies as for the child receiving it.
When it matters most
Overwhelmingly the first twelve weeks, when the eyes, ears and heart are being formed. After about twenty weeks the risk is very low.
CorrectNot this time
Many do — and stopping is often the greater risk.
Each medicine has to be judged separately, and the judgement is not simply does it cross. An untreated seizure, an untreated infection or uncontrolled asthma can be far more dangerous to a pregnancy than the drug that prevents it. This is precisely why the decision belongs to a doctor or pharmacist who knows the case, and why nobody should stop a prescribed medicine on their own.
When it matters most
Weeks three to eight are the most sensitive for structural effects, which is why the first appointment asks what someone is already taking.
CorrectNot this time
No — not in any useful amount.
Insulin is a protein, and proteins are enormous compared with alcohol or caffeine. The placenta lets small dissolved molecules through and effectively does not pass this one, which is exactly why insulin is the standard treatment for diabetes in pregnancy: it controls the mother’s glucose without reaching the foetus. It is the clearest proof that the rule is about size, not about safety — nothing has decided that insulin should be kept out.
When it matters most
Not applicable: it does not arrive. Glucose, on the other hand, crosses freely, which is why controlling the mother’s blood glucose still matters a great deal.
Timing · three windows
The same exposure does different things in different weeks.
- Weeks 1–2 · before most people knowThe ball of cells divides, implants and begins to build the placenta. There are as yet no organs to damage.Very little, or everything. An exposure at this stage tends either to have no lasting effect at all, or to stop the pregnancy continuing.
- Weeks 3–8 · the embryoHeart, brain, spine, limbs, eyes and ears are all laid down, in an organism smaller than your thumb.Structure. This is the window in which an exposure can change how an organ is built rather than how well it works — and it is largely over by week eight.
- Weeks 9–40 · the foetusThe organs that exist grow and mature. The brain keeps developing right up to birth and beyond.Growth and function. Birth weight, lung readiness and brain development — changes in how well something works rather than whether it formed.
Key fact
Whether a substance reaches the foetus depends on the size of its molecules and whether they dissolve in blood — not on whether it is harmful. The placenta has no filtering ability. What the substance then does depends on what is being built that week.
Think again
“The placenta filters out anything harmful.”
It filters nothing. Read the word barrier — which is how the placenta is often described — and it is easy to picture a sieve deciding what to admit. What is actually there is a very thin, very large surface designed to let small dissolved molecules through as fast as possible, because oxygen and glucose have to arrive at the rate a growing organism uses them. Anything else small enough and soluble enough gets the same easy passage: it is using the door built for the food. The one real rule is size. Large molecules such as insulin do not cross in any useful quantity, which is why insulin can be used to treat diabetes during pregnancy; antibodies are large too, and cross only because the placenta spends energy carrying them over deliberately. Everything else is physics, and physics has no opinion about what is good for a foetus.
“So anything that goes wrong is the mother's fault.”
That does not follow from any of the biology on this page, and it is worth saying why. Much exposure is not a choice: second-hand smoke, air pollution at the roadside, an infection caught from someone else, a medicine that must be continued because stopping it would be more dangerous than taking it. Some are choices constrained by circumstances — what food is affordable and near enough to buy. Addiction is a condition of the nervous system rather than a decision repeated daily, which is the point What drugs do to the body spends a whole lesson establishing. And a great many pregnancy complications have no identified cause at all. There is also the shape of the evidence: what studies measure is risk — a probability across large numbers of pregnancies — so an exposed pregnancy that comes to no harm and an unexposed one that does are both entirely ordinary results. Risk is a statement about populations. It is not a verdict on a person.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Apply the rule
Why does alcohol reach a developing foetus?
Rung 2 · The one that catches people
Which of these does NOT cross the placenta in any useful amount?
Rung 3 · Explain the rule
Explain what decides whether a substance reaches a developing foetus. Give two examples that cross and one that does not, and say why the placenta cannot protect the foetus from harmful substances.
Rung 4 · Take it somewhere new
A headline says: “Coffee in pregnancy harms babies.” Using what you know about crossing, dose and evidence, set out three questions you would need answered before deciding what to make of it.
Key note
The placenta lets small dissolved molecules diffuse across, and cannot distinguish useful ones from harmful ones. Alcohol, nicotine, carbon monoxide, caffeine and most medicines all reach the foetus; large molecules such as insulin do not. Carbon monoxide from smoke reduces the oxygen the blood can carry, which is why smoking lowers birth weight. Alcohol affects the developing brain and nervous system throughout. Organs are laid down in weeks three to eight, so that window matters most for structure, and the brain is being built for the whole pregnancy. Risk is a probability across many pregnancies, not a certainty about one.
Going further
Almost everything on this page is known because of thalidomide. It was sold from 1957 as a safe sedative, prescribed widely for morning sickness, and marketed as harmless partly because a large dose did not kill laboratory animals — which was, at the time, most of what safe testing meant. Thousands of babies were born with severely shortened or absent limbs. Two doctors working separately, William McBride in Australia and Widukind Lenz in Germany, traced the pattern back to the drug, and it was withdrawn in 1961. What made the case scientifically decisive was the timing: the damage depended on which days of the pregnancy the tablets were taken, in a window of only a few weeks while the limbs were forming, and the specific malformation shifted with the day. That is how precisely the sensitive window of development came to be mapped. The disaster is the reason drugs are now tested on pregnancy specifically, why licensing exists in the form it does, and why a pharmacist will not hand over an ordinary medicine to someone who is pregnant without checking first. Thalidomide itself is still in use today, under tight controls, for conditions including a cancer of the blood — which is the second lesson: the substance was never the villain, the missing knowledge was.
Before this lesson
- This follows on from Gestation, the placenta and birth.
Connects to
At GCSE this becomes
- Substances that affect development, the epidemiology behind public health advice, and how risk is estimated from large studies.
Where to next
Ask Mr Badmus AI
Want to check why molecule size decides what crosses?
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.
This lesson describes exposures and risk. Risk is a probability measured across large numbers of pregnancies, not a prediction about any one of them, and nothing here is a judgement about any person or family. Anyone who is pregnant or might be should speak to a midwife, doctor or pharmacist before starting or stopping anything, including prescribed medicines; that advice is free and confidential. Relationships and consent are covered in your RSE and PSHE lessons.
Lesson content © MrBadmusAI.