Life begins with conception: the union of egg and sperm.
Conception starts with one sperm cell entering an egg cell. As you can see on the picture a sperm cell is very small in comparison to the egg cell.
This fertilized egg cell moves through the fallopian tubes to the womb.
As soon as it is implanted in the lining of the uterus the girl is pregnant.
If everything goes well a child will be born 9 months later.
When you have sex you can prevent conception by using contraception.

How does it start?