Human reproduction

Human reproduction is the biological process by which a new human being develops from the union of two specialised cells, the gametes: a sperm and an egg. It is sexual reproduction, in which each gamete provides half of the genetic information. Understanding how it works is the basis for understanding contraception, fertility and reproductive health.

The reproductive systems

Each person has a set of organs that produce gametes and hormones, make it possible for gametes to meet and, in some cases, carry a pregnancy. Two configurations are usually described, although anatomy varies, as our entry on biological sex explains.

Male reproductive system

The testicles produce sperm and hormones such as testosterone. Sperm mature in the epididymis and travel along the vas deferens. The seminal vesicles and prostate add fluids that, together with the sperm, form semen, which leaves the body through the urethra in the penis.

Female reproductive system

The ovaries hold the eggs and produce hormones such as oestrogen and progesterone. The fallopian tubes pick up the egg and carry it towards the womb; this is usually where fertilisation happens. The uterus (womb) is where the embryo implants and develops, and the vagina connects it to the outside through the cervix.

The gametes

• Sperm: tiny, mobile cells with a tail that lets them swim. They are produced continuously from puberty onwards.
• Eggs: much larger cells that cannot move by themselves. Usually, one matures and is released each cycle.

Each gamete carries 23 chromosomes, half of the 46 in a human cell. When they join, the full number is restored. The sperm contributes either an X or a Y chromosome, and that combination (XX or XY) usually guides the embryo’s sexual development.

The menstrual cycle and ovulation

The menstrual cycle is a series of hormonal changes that prepare the body for a possible pregnancy. It is counted from the first day of a period. According to the NHS, the average cycle lasts about 28 days, but regular cycles of 21 to 35 days are normal.

• In the first part of the cycle, an egg matures in the ovary and the endometrium (the womb lining) thickens.
• Ovulation does not always fall “mid-cycle”: it usually happens 10 to 16 days before the next period.
• If there is no pregnancy, the lining is shed: this is the period, which starts a new cycle.

The fertile window

The egg survives for about 12 to 24 hours after ovulation, but sperm can survive for up to 7 days inside the body. That is why pregnancy is possible after sex several days before ovulation. Because cycles vary, there is no completely “safe” day for unprotected sex if you do not want to get pregnant – that is what contraceptive methods are for. If you are trying to conceive, our fertility and conception guide explains how to spot your most fertile days.

Fertilisation and implantation

Fertilisation is the union of a sperm with the egg, normally in the fallopian tube. Many sperm get close, but only one gets in. The result is a single cell called a zygote, which starts dividing as it moves towards the womb. About a week later, the cluster of cells attaches to the womb lining: this is implantation. If one zygote splits in two, identical twins develop; if two separate eggs are fertilised, the twins are non-identical.

Pregnancy by trimester

Clinically, pregnancy is dated from the first day of the last period, not from fertilisation. That is why it lasts around 40 weeks, divided into three trimesters:

• First trimester (up to week 13): the main organs form. Around week 10, the embryo starts to be called a fetus.
• Second trimester (weeks 14 to 27): the fetus grows quickly and its first movements are usually felt.
• Third trimester (week 28 to birth): the fetus gains weight and matures to live outside the womb.

The placenta and umbilical cord allow oxygen and nutrients to pass between the pregnant person and the fetus. Our pregnancy and postpartum guide covers care at each stage.

Birth

Birth between weeks 37 and 42 is considered full term; before 37 weeks it is premature, and contractions at that stage need urgent medical advice. In the first stage of labour, the cervix softens and opens; then contractions help the baby to be born through the vagina, and finally the placenta is delivered. When there are medical reasons, the baby is born by caesarean section.

Common myths and when to get advice

• “You can’t get pregnant right after your period”: it is unlikely but possible, especially with short cycles.
• “Ovulation is always on day 14”: only in some 28-day cycles; it varies between people and from month to month.
• “If it doesn’t happen straight away, something is wrong”: most couples conceive within the first year.

The WHO defines infertility as not achieving a pregnancy after 12 months or more of regular unprotected sex, and estimates that it affects around one in six people of reproductive age during their lifetime (2025). Untreated STIs such as chlamydia are a preventable cause of blocked fallopian tubes. See a doctor sooner if you are 35 or over, have very irregular or absent periods, or severe pelvic pain.