Many changes happen to the womb during pregnancy. A woman's womb is where the baby grows and develops, forcing it to grow and change in size. The muscles and blood vessels surrounding the womb also undergo changes related to the development of the fetus. When a woman goes into labor, the womb contracts and works the muscles to help push the baby through the birth canal.
The most significant and obvious change to the womb during pregnancy is the major change in size as the pregnancy progresses. The uterus begins to expand and fill the pelvis during the first trimester and continues to expand throughout the rest of the pregnancy. As the fetus grows in the womb during pregnancy it pushes up beneath the ribcage, pushing the other organs and tissues in the woman's abdomen slightly to the sides. The lower segment of the womb also forms during pregnancy, giving a place for the cervix to dilate during labor. Just before delivery, the uterus drops slightly to prepare the body for giving birth.
A lining of mucus forms on the inside walls of the uterus and is flushed away monthly when a woman is not pregnant. This lining stays inside the womb during pregnancy and provides a place for the placenta to attach to nourish the fetus and provide it with oxygen. The placenta detaches after a woman gives birth, and womb contractions after delivery push the placenta out through the birth canal.
The womb also holds the amniotic sac, which encases the fetus with amniotic fluid. This fluid maintains the fetus's body temperature and protects it from harm as it moves. The amount of amniotic fluid increases steadily throughout pregnancy as the baby grows. Toward the end of a pregnancy, most women carry about 1 liter (33.5 fluid ounces) of amniotic fluid.
Uterine muscles that support the womb grow and expand around the womb during pregnancy. The muscle fibers help hold the womb in place and stretch to accommodate the womb's change in size. As the muscles grow and stretch during pregnancy, they intertwine to form a net that supports the weight of the fetus, placenta, and fluids inside the womb. These same muscles contract and retract the cervix when the woman goes into labor to help push the baby out of the womb and down through the vagina. Blood vessels inside the uterus also dilate and increase during pregnancy.