Fertilisation occurs when a sperm and egg join to form an embryo. An embryo develops into an unborn baby in the uterus during pregnancy. In humans, conception begins when a sperm fertilises an egg.
When the uterus sheds its lining, it causes a flow of blood called a period. The first day of bleeding is day one of the cycle. Days five to seven, the bleeding slows and stops and the uterus ...