Reproductive System

The reproductive system is the group of body parts that allows humans to make babies. It is the only body system that does not start working at birth. Instead, it stays quiet during childhood and turns on during puberty, usually between ages 9 and 15. Once it is active, the reproductive system can join with another person's reproductive system to create a new human being.

Humans have two main types of reproductive systems. Doctors call them the female system and the male system. Most girls are born with one type, and most boys are born with the other.

The female reproductive system has several parts inside the body. The two ovaries are about the size of almonds. They store tiny cells called eggs. A baby girl is born with about one to two million eggs already inside her ovaries, more than she will ever use. Once a month after puberty, an ovary releases one egg. The egg travels down a tube called a fallopian tube. It ends up in the uterus, a muscular pouch about the size of a small pear. If the egg is not joined with a sperm cell, the body washes it away. This monthly cycle is called menstruation, or a period.

The male reproductive system has parts both inside and outside the body. The two testicles make tiny cells called sperm. A healthy adult makes hundreds of millions of new sperm cells every day. Sperm are some of the smallest cells in the human body. About 175,000 of them lined up would stretch only one inch.

A new baby starts when one sperm cell joins with one egg cell. This joining is called fertilization. The new single cell holds DNA from both parents. Half the instructions come from the mother. Half come from the father. That is why kids often look a little like each parent. The single cell then splits into 2, then 4, then 8, then more, growing inside the uterus. About 40 weeks later, a baby is born.

Other body systems matter for reproduction too. The endocrine system sends out chemical signals called hormones that start puberty and run the monthly cycle. The brain controls the timing. Doctors who specialize in this system help people stay healthy, treat problems, and support people who want to have children. Every human alive started as a single fertilized cell smaller than a grain of sand.