How does the reproductive system work?

Every living thing makes more of its own kind. In humans, the reproductive system makes special cells called gametes: eggs in a female body and sperm in a male body. When one egg and one sperm join, a new life can begin. We are facing the person, so their right is on your left.

The female organs sit low in the belly, inside the bony ring of the pelvis. Two almond-sized ovaries store thousands of immature eggs and release about one a month. Each egg is caught by the finger-like end of a fallopian tube and swept towards the uterus (womb), a pear-sized bag of strong muscle where a baby can grow. Its narrow lower end is the cervix, which opens into the birth canal.

The male organs start with two testes, which make millions of sperm every day. They hang in a pouch outside the body because sperm need to stay 2 to 3 °C cooler than the rest of you. Sperm mature in the coiled epididymis, then travel up the vas deferens. The seminal vesicles and the prostate add fluids that feed and protect them, and the urethra carries them out.

Both sets of organs are also glands: the ovaries make oestrogen and progesterone, and the testes make testosterone. EndocrineClear shows how the brain controls them. The hip bones around them are drawn faintly; SkeletonClear has the real pelvis.

Puberty is the time when a child's body becomes an adult body that could one day have children. It does not start in the reproductive organs. It starts in the brain.

Deep in the brain, the hypothalamus begins to release a hormone called GnRH in little pulses, about every one to two hours, at first mostly at night. GnRH tells the pea-sized pituitary gland just below it to send two hormones into the blood: LH and FSH. They reach the ovaries, which begin to make oestrogen and progesterone, or the testes, which begin to make testosterone and sperm. EndocrineClear shows this chain of glands in depth.

These hormones change the whole body, step by step. Everyone has a growth spurt, grows hair under the arms and elsewhere, sweats more and may get oily skin and spots. In girls, breasts develop, hips widen and periods begin. In boys, the testes grow, the voice deepens and facial hair appears. Feelings can change quickly too, and that is part of it.

The timing varies a lot. Puberty usually starts somewhere between about 8 and 13 for girls and 9 and 14 for boys. Starting early or late within those ranges is normal, and bodies of every shape are normal. If it starts before 8 (girls) or 9 (boys), or hasn't started by about 13 or 14, a doctor can check. Growth stops when the growth plates in the bones close (see SkeletonClear).

From puberty until about age 50, the ovaries and uterus follow a repeating menstrual cycle. We count it from day 1, the first day of a period. A cycle usually lasts 21 to 35 days; 28 is only an average, and in the first few years cycles are often irregular.

Days 1 to about 5: the thick lining of the uterus, which was not needed, comes away with a little blood and leaves through the cervix and birth canal. That is the period. It usually lasts 2 to 7 days, and the total blood lost is only a few spoonfuls, about 30 to 40 mL.

The follicular phase: FSH from the pituitary makes a follicle, a tiny fluid-filled bubble around an egg, grow in the ovary to about 2 cm. It makes oestrogen, which rebuilds the lining.

Ovulation, about 14 days before the next period (day 14 of a 28-day cycle): a sudden surge of LH makes the follicle burst and release its egg. The fallopian tube catches it. The egg lives for only about a day.

The luteal phase: the empty follicle becomes the corpus luteum ("yellow body") and makes progesterone, which makes the lining thick, soft and full of blood vessels, ready for a baby. If no baby starts, the corpus luteum fades, progesterone falls, and the lining is shed: day 1 again.

Periods are normal and healthy, and nothing to be ashamed of. Use a clean pad, cloth pad, cup or tampon; change pads about every 4 to 6 hours; wash your hands; wrap used pads and put them in a bin; wash cloth pads with soap and dry them in the sun. In India, about 77% of young women used a hygienic method in 2019–21 (NFHS-5). You can go to school, play sport and do everything as usual.

This happens at the scale of cells, far too small to see with your eyes. The egg is about 0.1 mm across, the biggest cell in the human body, just about the size of the full stop at the end of this sentence. A sperm is much smaller: a head about 5 µm long and a long tail it whips to swim.

After ovulation, the egg is swept into the wide part of the fallopian tube. It can be fertilised for only about 12 to 24 hours. Of the millions of sperm that set out, only a few hundred reach the egg. They push through the crown of cells around it, and just one gets through the clear coat, the zona pellucida, and joins the egg. At once the zona hardens so that no other sperm can get in.

Now the egg is a zygote: a single new cell. The egg brought 23 chromosomes and the sperm brought 23, so the zygote has 46, in 23 pairs, the full set of instructions for a new person. Every egg carries an X chromosome. About half of sperm carry an X and half a Y. XX usually makes a girl and XY a boy, so the baby's sex is set by chance, by the sperm, never by the mother.

As it drifts down the tube, the zygote divides: 2 cells at about 30 hours, then 4, 8 and 16, without growing any bigger. By about day 4 it is a ball of cells in the uterus. It becomes a hollow blastocyst, with an inner cluster that will become the baby. Around days 6 to 10 it hatches out of its coat and implants, burrowing into the thick lining that the cycle prepared.

Doctors count a pregnancy from the first day of the last period, so it lasts about 40 weeks, or 280 days, even though fertilisation happens around week 2. For the first 10 weeks the growing baby is called an embryo; after that, a fetus.

Everything starts tiny. At week 6 the embryo is the size of a lentil, yet its heart is already beating. By week 12 it is the size of a lemon and has fingers and toes. By week 20 it is as long as a banana and the mother can feel it kick. At week 40 a baby is usually about 50 cm long and weighs about 3 to 3.5 kg. The uterus grows with it, from pear-sized to reaching up near the ribs.

The baby floats in amniotic fluid inside a sac, which cushions it and keeps it warm. It cannot eat or breathe air, so it gets everything through the placenta, a disc that grows on the wall of the uterus, and the umbilical cord. In the placenta, the mother's blood and the baby's blood flow very close together but do not mix: oxygen and food pass into the baby's blood, and carbon dioxide and waste pass back. The cord carries it all in one vein and two arteries. CirculationClear shows the baby's own circulation and how it changes with the first breath.

Birth: at about 40 weeks, the muscle of the uterus squeezes in waves (labour). The cervix opens to about 10 cm, and the baby is pushed out through the birth canal, usually head first; the placenta follows. Some babies are born by an operation called a caesarean section.

Like every part of the body, the reproductive system can need care. This chapter explains common things; it cannot tell you what is happening in your own body. For that, see a doctor. Doctors see these questions every day, so there is nothing to be embarrassed about.

Care in pregnancy keeps mothers and babies safe. Regular check-ups catch problems such as high blood pressure and anaemia early. In India, JSY (Janani Suraksha Yojana, 2005) helps mothers give birth in a hospital, and PMSMA (2016) offers a free check-up by a doctor on the 9th of every month. Partly thanks to this, the share of mothers who die in childbirth has fallen by more than three-quarters since the late 1990s.

Menstrual health: periods that are very heavy, very painful, very irregular or missing are worth a doctor's visit. PCOS, a common hormone condition, is one reason periods can be irregular. Eating iron-rich food helps prevent anaemia.

Infections of the reproductive organs are common and most can be treated; signs like itching, pain, sores or unusual discharge mean: see a doctor early. The HPV vaccine prevents most cervical cancer. Infertility affects about 1 in 6 adults, can come from either partner, and is often treatable, including by IVF.

Respect: your body belongs to you, and everyone else's body belongs to them. Always ask, always respect a "no", and tell a trusted adult if anything ever feels wrong.