A baby inside its mother's uterus at around 12 weeks after fertilization The key feature of all living things is that they can make more of their species to replace those that have died. This is called reproduction. The human body reproduces in much the same way as animals such as cats, dogs, horses and tigers. A female and male come together and have sexual intercourse. The parts of the body involved in reproduction are called the sex organs. A microscopic, tadpole-shaped cell from the male joins with, or fertilizes, an egg cell from the female. Over the next nine months, the fertilized egg divides repeatedly, growing and developing inside the mother’s uterus to produce a baby ready to be born. This time is known as pregnancy.
The sex organs are present at birth, but they develop rapidly and start to work from the ages of about 11–14 years in girls and 13–16 years in boys. This time of rapid growth and changes in bodily features is known as puberty. A girl develops a more rounded body outline and her breasts enlarge. A boy grows facial hair, his voice breaks and his Adam's apple (a bump under the skin of his neck) increases in size. These changes are controlled by hormones.
If a baby were to continue to grow at the same rate as it did in the womb, it would reach a height of two kilometres (one and a quarter miles) by the time it is one year old.
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