When you become pregnant, the part of your body affected first and the part that undergoes the most significant changes is the uterus. It increases to 20 times its original weight, and 1,000 times its initial capacity. The amount of its muscle, connective and elastic tissue, blood vessels, and nerves increases. Its shape changes from elongated to oval by the second month, to round by midgestation, then back through oval to elongated at term (the end of a normal nine-month pregnancy).
The uterus softens beginning at the sixth week. It changes position as it increases in size, ascending into the abdomen by the fourth month and eventually reaching to the liver. It also becomes more contractile (the tendency to decrease in size), with irregular, painless Braxton Hicks contractions beginning in the first trimester. You may feel these contractions in the last weeks of pregnancy, when they are known as false labor.
Other parts of the reproductive system change along with the uterus. The cervix and vagina have an increased blood supply, which causes a darkening in color apparent by the sixth week. The amount of elastic tissue increases to prepare the way for the stretching that will be required during delivery. Secretions increase, and a mucous plug develops in the cervix. The fallopian tubes, ovaries, and ligaments supporting the uterus all enlarge and elongate. The ovaries, of course, cease to ovulate.
During the fourth month, the uterus grows into the abdomen, causing the abdominal wall to expand to accommodate it. The connective and elastic tissues stretch and straighten, creating thinned areas called striae (stretch marks). While the red of the striae may fade, silver remnants usually remain after delivery. In 50 percent of women, striae develop in the third trimester. Late in pregnancy, the internal pressure from the large uterus may even cause the muscles of the abdominal wall to separate (diastasis).
Your breasts must undergo many changes during pregnancy to produce milk. In the first two months of pregnancy, your breasts may feel sore or full. They enlarge, and veins may become visible. Striae can develop. The nipples also increase in size and usually darken. By midpregnancy, colostrum (a thick, yellowish fluid) can be expressed, but milk is not produced until after delivery.
Blood flow also changes during pregnancy, in order to satisfy your enlarged reproductive organs and feed your baby. Read about changes in the blood during pregnancy next.