About nine months before you were born, you started to grow from just two cells. One cell, a sperm cell, came from your father. The other cell, an egg cell, came from your mother.
These two cells joined together inside your mother’s body and formed a new cell called a fertilized egg. This cell was the very beginning of a whole human being, you.
When you were only a fertilized egg, you were about the size of the head of a pin!