During pregnancy, a developing fetus stays inside the uterus of the mother. The uterus is a muscular organ located in the pelvis, and it is designed to protect and nourish the growing fetus. The inner lining of the uterus, called the endometrium, thickens and becomes richly supplied with blood vessels in preparation for the implantation of a fertilized egg. Once the egg implants in the endometrium, the placenta develops to provide the fetus with oxygen and nutrients from the mother's bloodstream. The amniotic sac, which is filled with amniotic fluid, surrounds and cushions the fetus, protecting it from injury and allowing it to move freely.