During pregnancy, the uterus undergoes significant changes in size, shape, and function to accommodate the growing fetus. The muscles, blood vessels, and amniotic sac all expand and contract to support the weight of the baby and prepare for delivery.
Many changes occur in the uterus during pregnancy. A woman’s uterus is where the baby grows and develops, forcing it to grow and change in size. The muscles and blood vessels surrounding the uterus also undergo changes related to the development of the fetus. When a woman goes into labor, the uterus contracts and works muscles to help push the baby through the birth canal.
The most significant and obvious change to the uterus 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 uterus during pregnancy, it pushes under the rib cage, pushing the other organs and tissues of the woman’s abdomen slightly to the side. The lower segment of the uterus also forms during pregnancy, allowing the cervix to dilate during labor. Just before delivery, the uterus lowers slightly to prepare the body for delivery.
A coating of mucus forms on the inner walls of the uterus and is rinsed out monthly when a woman is not pregnant. This lining stays inside the uterus during pregnancy and provides a place for the placenta to attach to to nourish the fetus and provide it with oxygen. The placenta detaches after a woman gives birth, and contractions of the uterus after delivery push the placenta out through the birth canal.
The uterus also contains the amniotic sac, which encloses the fetus with amniotic fluid. This fluid maintains the body temperature of the fetus and protects it from damage as it moves. The amount of amniotic fluid increases steadily throughout pregnancy as the baby grows. Towards the end of a pregnancy, most women carry around 1 liter (33.5 fluid ounces) of amniotic fluid.
The uterine muscles that support the uterus grow and expand around the uterus during pregnancy. Muscle fibers help hold the uterus in place and stretch to accommodate the change in size of the uterus. As muscles grow and stretch during pregnancy, they intertwine to form a network that supports the weight of the fetus, placenta, and fluids within the uterus. These same muscles contract and retract the cervix as a woman goes into labor to help push the baby out of the uterus and down through the vagina. The blood vessels inside the uterus also dilate and enlarge during pregnancy.
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