A baby develops in the uterus, or womb a hollow, muscular organ in the mother's abdomen. The period of development in the uterus lasts about nine months in most cases. During this period, development is more rapid than at any time after birth For a baby to develop, a sperm from the father must unite with an egg from the mother. This union of a sperm and an egg is called fertilization. It produces a single cell called a fertilized egg. By a series of remark able changes, the fertilized egg gradually develops into a baby.