Where babies come from
Sperm and egg, one cell becomes many, nine months, and birth
Half a recipe from each parent
Almost every cell in your body carries a full recipe for you, written on 46 chromosomes. But sperm and egg cells are special: each one carries only half, 23 chromosomes. When a sperm and an egg join, the two halves make one full recipe. That is why you are a mix of both your parents.
Body cells are diploid: 23 pairs of chromosomes, one of each pair from each parent. Sperm and eggs are made by a special kind of cell division, meiosis, which shares out one chromosome from each pair, so each sperm or egg is haploid (23). Which one of each pair goes into a given sperm or egg is down to chance, and the pairs also swap pieces first, so every sperm and egg is a different mix. That is why brothers and sisters (except identical twins) are not the same. The DNA lesson shows what a chromosome is made of.
Where sperm are made
Sperm are made in the testes, the two glands that hang in the scrotum below the penis. They make millions of sperm every day, and each one carries half a recipe. The sperm travel up a long tube, past two glands that add fluid, and out through the penis.
Sperm are made inside the testes (testicles) in coiled tubes, and take about two months to mature. They are stored in the epididymis, the comma-shaped coil on the back of each testis. From there each ductus deferens (vas deferens) carries them up out of the scrotum, into the pelvis, over and behind the bladder. The seminal glands (seminal vesicles) and the prostate add most of the fluid; together that is semen. The two ejaculatory ducts run through the prostate into the urethra, which carries semen, and at other times urine, out through the penis. The testes sit outside the body because making sperm works best a few degrees below body temperature. The testes also make the hormone testosterone.
Where the egg is made
Eggs are made in the ovaries, two small glands on each side of the uterus (the womb). About once a month, one ovary lets go of one egg. The frilly end of a tube catches it, and the egg drifts slowly along the tube towards the uterus. The egg is the biggest cell in the body, but it is still only just big enough to see.
A girl is born with all the eggs she will ever have, about 1 to 2 million, still unfinished. From puberty, roughly once every 28 days (it varies a lot) hormones ripen one egg and the ovary releases it: ovulation. Over a lifetime only about 400 or so are released. The fimbriae, the finger-like fringe on the end of the uterine (fallopian) tube, sweep the egg in, and tiny hairs and muscle in the tube carry it towards the uterus over a few days. The egg lives for only about a day after release. Meanwhile the lining of the uterus has thickened, ready. If the egg is not fertilised, the lining is shed: that is a period. A human egg is about 0.1 mm across. The bones and muscles in this app come from one male scan; the female organs are from the Visible Human Female, fitted onto the same pelvis, so they are not from the same person.
Sperm swim to the egg
Sperm get into the vagina and swim to the egg. They swim up through a small opening, the cervix, across the uterus and into the tubes. Millions set off, but only a few hundred get near the egg. The egg waits in the widest part of the tube.
Each ejaculation holds tens of millions of sperm or more. Most never get past the cervix, and many swim up the tube that has no egg in it, so only a few hundred reach the egg. Fertilisation usually happens in the ampulla, the wide outer part of the uterine tube (lit here). Sperm can survive in the body for up to about 5 days, and the egg for about a day. The bones and muscles in this app come from one male scan; the female organs are from the Visible Human Female, fitted onto the same pelvis, so they are not from the same person.
They meet
Many sperm crowd round the egg, but only one gets in. The moment it does, the egg’s shell changes so no other sperm can get in. The 23 chromosomes from the sperm and the 23 from the egg come together: now it is one cell with the whole recipe, 46.
This is fertilisation. The sperm’s head releases enzymes to get through the egg’s outer layers, including the zona pellucida, a clear shell of protein. When one sperm fuses with the egg, the egg releases granules that change the zona so no other sperm can bind: this stops a second set of chromosomes getting in. The fertilised egg is a zygote. The sex chromosomes decide the baby’s sex: the egg always carries an X; the sperm carries either an X (making XX, usually a girl) or a Y (making XY, usually a boy). In the drawing a sperm is shown at roughly true size next to the egg: its head is about 5 thousandths of a millimetre long.
One cell becomes many
The single cell splits into two, then four, then eight. Each time, every new cell gets a full copy of the same recipe. While it splits, it rolls slowly down the tube. After about five days it is a hollow ball of cells, and about a week after the egg met the sperm it sinks into the soft lining of the uterus.
The first divisions (cleavage) happen inside the zona pellucida, so the ball does not grow: the cells just get smaller. Before each split the cell copies all its DNA, the way the DNA lesson shows, so every cell carries the same 46 chromosomes. By about day 4 it is a solid ball of 16 or so cells (a morula), and by day 5 a hollow ball, the blastocyst: an outer layer that will become part of the placenta and a small clump inside, the inner cell mass, that will become the baby. It hatches out of the zona and, from about day 6, buries itself in the uterus lining: implantation. It is so small that a cell here is drawn about a thousand times bigger than life.
Week 4: a poppy seed
Doctors count pregnancy from the start of the last period, so the egg met the sperm about two weeks ago. The ball has dug into the uterus wall. Inside a tiny bubble of fluid, a curled-up embryo about 2 mm long has begun, with a little yolk sac beside it.
Counting from the first day of the last period is called gestational age; a full-term pregnancy is about 40 weeks by this count, 38 weeks from fertilisation. At week 4 the embryo is protected by the amniotic sac, filled with cushioning fluid, and attached to the yolk sac, which feeds it until the placenta takes over. The outer layer of cells is growing into the placenta. The heart, brain and spinal cord are among the first things to form. Not drawn to scale with the next steps: the camera zooms in on the embryo.
Week 8: a raspberry
Now the embryo is about 16 mm long. Its heart has been beating for a couple of weeks, very fast, about 150 times a minute: the red dot. Little arms and legs are growing, with the arms ahead of the legs. The head is huge compared with the body.
The heart starts beating at a regular rhythm around weeks 6 to 7 and can be seen on an ultrasound scan. The upper body grows faster than the lower, so the arms are longer than the legs; knees, ankles and toes are not there yet. The umbilical cord now joins the embryo to the growing placenta on the uterus wall. Around the end of week 10 the embryo is called a fetus (foetus). The heart rate is about twice an adult’s.
Week 12: a plum
The baby is 5.4 cm from head to bottom and looks like a tiny person. All the main organs and muscles are there and growing. The placenta, the dark red disc on the uterus wall, now feeds the baby through the cord.
The placenta grows into the uterus wall. The mother’s blood flows past it and the baby’s blood flows through it, and the two are kept separate by a very thin barrier: food and oxygen pass into the baby’s blood, and waste like carbon dioxide passes back into the mother’s to be breathed out and cleaned away. The two bloods do not normally mix. The umbilical cord carries the baby’s blood to and from the placenta in two arteries and one vein. The skeleton is still mostly soft tissue, slowly hardening into bone. Uterus sizes in these drawings are rough.
Week 20: a banana
Halfway there. The baby is about 25.6 cm from head to heel. It kicks, punches, turns round and sucks its thumb, and the mother may start to feel it moving, like bubbles or fluttering. The top of the uterus has reached about the level of her belly button.
From about week 20 the NHS measures babies from head to heel instead of head to bottom, because the legs are long enough. Thumb-sucking practises the sucking reflex needed to feed after birth. The baby floats in amniotic fluid, which it swallows and breathes in and out; its lungs are not used for air until it is born, because the placenta supplies all its oxygen.
Week 40: ready to be born
After about nine months the baby is around 51 cm long and fills the uterus. Most babies have turned head-down by now, ready to come out head first. The uterus has grown from the size of a plum to reach almost up to the ribs.
Full term is 37 to 42 weeks; 40 weeks is only the average, and few babies arrive on their due date. By week 40 it has been about 38 weeks since fertilisation. Most babies settle head-down (cephalic) by the last few weeks. A newborn usually weighs around 3 to 4 kg. Twins happen in two ways: one fertilised egg that splits into two (identical twins, the same recipe) or two eggs fertilised by two sperm (non-identical twins, as alike as any brother or sister).
Birth
The uterus is a big muscle. When the baby is ready, it starts to squeeze, again and again. These squeezes are contractions. They slowly open the cervix until it is about 10 cm wide. Then the muscle pushes the baby out through the vagina, head first. It usually takes many hours. Then the baby takes its first breath.
Labour usually starts with contractions that come every few minutes and get stronger, longer and closer together; the NHS says to call the midwife when they come every 5 minutes or more often. In the first stage the cervix softens, thins and opens; established labour starts at about 4 cm and it is fully open at about 10 cm, which for a first baby usually takes 8 to 18 hours. In the second stage the mother pushes and the baby is born; in the third, the uterus contracts again and the placenta comes out too (the "afterbirth"). The cord is clamped and cut, usually after a minute or more; it has no nerves, so it does not hurt, and what is left becomes the belly button. With its first breaths the baby’s lungs fill with air for the first time, and its blood starts going round through the lungs instead of to the placenta. Sometimes the baby is born by caesarean section instead: an operation that lifts it out through a cut in the belly and uterus. In the drawing hours are squeezed into seconds.