What to Expect in Changes to the Body During Pregnancy

Pregnancy is a time of great physical change in a woman's body. Not only is her body nourishing a growing new life, it's also preparing for the work of labor and breast-feeding after the birth. As a result, a pregnant woman can expect more than her belly to change.
Circulatory System
A woman's blood volume increases by up to 50 percent during pregnancy, and her circulatory system grows to accommodate this extra volume. This generally means that a woman's blood pressure will drop by several points in the middle of pregnancy, gradually returning to prepregnancy levels. Unfortunately, the increased blood volume also indirectly causes swelling in the extremities, particularly the legs and feet. Since the body needs extra fluid to make all that extra blood, it tends to store it in the tissues.
Respiratory System
Progesterone increases a pregnant woman's lung capacity, bringing in 30 percent to 40 percent more air in order to carry more oxygen to the baby. Pregnant women often breathe faster than normal and feel they are out of breath (even though they're not), starting at around the beginning of the second trimester. The rib cage also enlarges by a few inches to accommodate the increased lung capacity.
Digestive System
Pregnancy hormones slow digestion during pregnancy to better absorb nutrients. Unfortunately, this often results in heartburn and constipation. These are among the most common pregnancy complaints.
A woman's breasts increase in size during pregnancy. The nipples and areolas will also darken, and later in pregnancy the breasts may begin to leak colostrum (the baby's first milk).
Uterus and Belly
The uterus houses the fetus throughout pregnancy and grows along with it, eventually increasing to about 1,000 times its original size. As it grows, it will begin to crowd out other internal organs (the most noticeable of which will be the bladder, as this will result in the need to urinate more often). As it grows, the belly grows firm, then distends into the familiar "bump" of pregnancy.
Bones, Muscles and Joints
As pregnancy progresses, the ligaments in the abdomen will stretch and the joints in the pelvis will loosen, which can cause pelvic or back pain. As the belly grows, the spine will curve to compensate, which can also cause back pain.
The skin generally darkens in several areas during pregnancy--the areolas, linea nigra (a line running from the belly button down to the pubic hair), existing moles and freckles, the belly button, and face are very common. Some of the darkening will fade after delivery, although some areas will probably remain darker than they were before pregnancy.
Mayo Clinic Guide to a Healthy Pregnancy; Roger W. Harms, M.D., Robert V. Johnson, M.D., Mary M. Murry. C.N.M.; 2004