It all began when your mother had sex with your father. His penis entered her vagina. and when he came (reached his climax), his semen shot into her vagina.
In the semen were his sperm cells. The semen came from his prostate gland and the sperm cells came from his balls (testicles). Sperm cells are very small. You can only see them under a microscope. There are millions of sperm cells in one drop of semen. Sperm cells look a bit like tadpoles: a head and a long tail. The head is really the cell. The tail is only for moving.
Looking for an egg cell
The sperm cells swim from the vagina into the womb (uterus) and from there to the fallopian tubes. It is a difficult and dagerous jouney with many obstacles, and only a small number of sperm cells reach the fallopian tubes. There they hunt for an egg cell. Egg cells come from the overaries. Egg cells are much bigger than sperm cells; you can just see them with the naked eye. Egg cells attract sperm cells.
Every month an egg cell jumps from one of the ovaries into the fallopian tube. This is called ‘ovulation’ and it takes place two weeks before the next menstruation. (Read about this in Menstruation) When your were made, your mother had sex at the time of her ovulation or some days before that. So when the sperm cells arrived in the fallopian tubes, there was an egg cell in one of them. One of the sperm cells penetrated the egg cell and the genetic material of your father and mother came together. This is called ‘fertilisation’. The fertilised egg cell travelled in about a week to the womb and settled in the wall of the womb. You began to grow, getting your food and oxygen from your mother through the umbilical cord. (Read also How you grew inside your mother).