Human Reproduction

The male reproductive system has one big job: to make and deliver sperm, the male sex cells. Sperm are made inside two organs called the testes, which sit in a pouch of skin called the scrotum outside the body. This keeps them slightly cooler than the rest of the body, which is exactly the temperature sperm need to develop well. Each sperm cell is tiny, with a head carrying half the genetic instructions for a new person and a tail that whips back and forth to help it swim. As sperm mature, they travel through tubes and are mixed with fluids from glands, forming semen. The testes also make testosterone, a hormone that drives changes during puberty, like a deepening voice and new muscle growth, and keeps sperm production going throughout adult life.

The female reproductive system makes egg cells and provides a safe place for a baby to grow. Two organs called ovaries store the eggs. A female is born with all the egg cells she will ever have, and starting at puberty, about once a month one egg matures and is released in an event called ovulation. The egg is swept into a fallopian tube, a narrow tunnel that leads toward the uterus. Fertilisation, where a sperm joins an egg, normally happens inside this tube. The uterus is a strong, stretchy, muscular organ. Its lining thickens each month to get ready for a possible pregnancy, and if no egg is fertilised, the lining breaks down and leaves the body during a period. If a fertilised egg settles into the lining, the uterus becomes the baby's home for about nine months.

Humans reproduce sexually, which means a new person starts when two special cells called gametes join together. The male gamete is the sperm, and the female gamete is the egg. Each gamete carries exactly half the normal number of chromosomes, so when they combine at fertilisation, the new cell has a complete set, half from each parent. Males make sperm in the testes through a process called spermatogenesis, which begins at puberty and can continue for the rest of their lives, producing millions of new sperm every day. Females make eggs through oogenesis, which actually starts before they are even born. All the eggs a female will ever have form early, then pause, and from puberty onward usually one egg finishes maturing each month. This difference explains why males constantly make fresh gametes while females work from a stored supply.

Conception is the moment a sperm cell joins an egg cell and a new life begins. An egg is released from an ovary and enters a fallopian tube, where it can survive for about a day. Millions of sperm swim up through the uterus and into the tubes, but only a few hundred ever reach the egg, and usually just one sperm breaks through the egg's outer layer. The instant that happens, the egg seals itself so no other sperm can enter. The genetic material from the sperm and egg merge, forming a single new cell called a zygote with a full set of chromosomes. Over the next few days the zygote divides again and again as it travels to the uterus, where it settles into the soft lining and starts to grow into an embryo.

From the ninth week of pregnancy until birth, the developing baby is called a fetus. During this time it grows from about the size of a cherry to a full newborn, and all its organs finish forming and start working. The fetus cannot eat or breathe on its own, so it depends completely on the placenta, a special organ that grows in the wall of the uterus. The placenta connects to the fetus through the umbilical cord and acts like a delivery service and a filter: oxygen and nutrients pass from the mother's blood to the fetus, and waste products pass back to the mother to be removed. Because the fetus shares this supply line, what the mother eats, drinks, or breathes matters a lot. Healthy food helps the fetus grow, while alcohol, smoking, and certain drugs or infections can cross the placenta and harm development. As birth approaches, the fetus usually turns head down, ready to be born.