The uterus changes during pregnancy. It grows with the baby. It also has fluid and the placenta inside. It starts small but it becomes very large. It can hold as much as five to twenty liters. The uterus is made of smooth muscle called the myometrium. The cells of the myometrium grow during pregnancy. Strong fibrous tissue grows on the outside of the uterus. The myometrium grows at first but becomes thinner at the end of the pregnancy. A doctor or nurse can feel the baby through the thinning uterus.