The summary of pregnancy is that it is the process by which a baby grows and develops inside a woman's body before birth. The average length of pregnancy is 40 weeks, or just over 9 months. During this time, the baby develops from a fertilized egg, called a zygote, into a fully-formed baby. The baby grows inside the woman's uterus, and is nourished by the woman's placenta. At birth, the baby is delivered through the woman's vagina.