The procedure involves fertilizing an egg outside the body, in a laboratory dish, and then implanting it in a woman's uterus.

In a normal pregnancy a male sperm penetrates a woman's egg and fertilizes it inside her body after ovulation - when a mature egg has been released from the ovaries. The fertilized egg (now an embryo) then attaches itself to the wall of the uterus (womb) and begins developing into a baby. This is known as natural conception. 

However, some women are unable to become pregnant through natural or unassisted conception and have to undergo fertility treatment to become pregnant.