By the 20th week of pregnancy, your uterus can be felt at the level of your belly button (umbilicus). The pelvic colon and small intestines are crowded upward and backward. The ascending and descending colon maintain their usual positions. At this point, your uterus is especially enlarged where the placenta attaches to it (usually on the front or back wall). This gives the uterus an uneven bulge. The wall of the uterus, which lengthens and thickens early in pregnancy, stretches as the fetus grows, and becomes thinner now – just 3 to 5 millimeters thick. Your bladder moves up but not as much as your uterus, which straightens as it moves up. As your uterus moves up, it rests against the lower portion of the front of your abdominal wall, causing it to bulge forward noticeably by your 20th week. The size of the bulge depends on how strong your abdominal muscles are. If they are firm, the uterus may be pressed against the spinal column, and there will be no noticeable bulge; if they are weak, the pressure of the uterus against the inside wall makes a sizeable bulge. At this point, you should be able to feel light movements of the fetus. This is called “quickening.” You may recognize this earlier if you have been pregnant before. The fetus sleeps and wakes at regular intervals, is more active, is about 9 inches long and weighs between a half-pound and a pound.

At this point in pregnancy, the top of your uterus is about one-third of the distance between the bellybutton and the xiphoid cartilage at the lower end of your breastbone. Constipation is common because your uterus is pressing on your lower colon and hormones slow down your body’s excretion process. Between the growth of your uterus and general weight gain, you may be feeling fatigued. Some women also experience heartburn as your uterus presses against your stomach. Your breasts are also changing to get ready for breastfeeding. First colostrum and then milk are produced by the grape-like clusters of tiny sacs (alveoli) deep within the breast tissue. Clusters of alveoli form lobules, which come together to form 15 to 20 lobes. Each lobe connects to a lactiferous duct for conveying milk. As the ducts extend toward the nipple and areola (darker area around the nipple), they widen into the lactiferous sinuses. These sinuses (or milk pools) release the milk through 15 to 20 tiny nipple openings in each breast when the baby nurses. At week 28, the fetus is about 16 inches long and weighs two to three pounds. The skin is wrinkled but will become less so as more fat builds up under the skin in the next few weeks. Fine, downy hair called lanugo, and a waxy white protective substance covering the skin called vernix, are on the fetus’ body. Its eyes are open, and eyebrows and eyelashes were formed in the fourth month. The fetus sucks its thumb and its taste buds have developed. It kicks, stretches and moves frequently in your uterus—you’ll feel it moving around and others might even be able to see these movements! Fetal organs and systems are quite well developed by the 28th week of pregnancy, but the final two months of gestation are important for further maturation of all body systems and organs.

By the end of the 36th week of pregnancy, your enlarged uterus almost fills the space within your abdomen. The fetus is inside the membrane sac within the uterus and high within the abdomen. The muscles of your abdomen support much of its weight. During this week, the top of the uterus is at the tip of the xiphoid cartilage at the lower end of the breastbone, which is pushed forward. The change in the position of the heart and the upward pressure of the diaphragm may make it hard to breathe at this point. The crowding of your stomach and intestines may contribute to discomfort after eating. Your cervix is long, thick and filled with the mucous plug. By the 36th week, your vagina and urethra are elongated and all the tissues in the perineum (area between vaginal and anal openings) are enlarged. The swollen perineum projects outward in the last weeks of pregnancy and readily expands during labor. The brain of the fetus is growing rapidly, but bones in the skull are soft so that he or she will fit through your vagina at birth. The lungs are still forming. You will likely feel the fetus kicking and may be aware of rhythmic movements, which could be hiccups or thumb sucking. Another possible sensation, sudden movement, may be a startle response.

At full term, or 40 weeks of pregnancy, the fetus’ head has generally lowered into your pelvis, where it takes up most of the space. This is called “lightening.” In first pregnancies, this may happen a few weeks before labor. In repeat pregnancies, this can happen at the time of labor. The canal of the broad, enlarged cervix is still filled with the plug of mucous. If this is your first pregnancy, the small opening at the bottom of your cervix is usually not dilated, whereas if you have given birth before, it will often be open as wide as two fingers some time before labor begins. At this point, you may be experiencing frequent urination, increased constipation, edema (water retention) and aching legs or vulva. Varicose veins in the vulva, rectum and legs are also possible. This is because of the position of the uterus, the pressure of the baby's head and a loss of muscle tone as the hormone relaxin loosens your tissues in preparation for birth. Other changes at this time include increased development of blood vessels and increased amount of blood.

You can see that the round ligament is long and enlarged. It is also farther forward because of the twisting of the uterus. The enlarged uterosacral ligament is shown stretched taut by the enlarged uterus. Backaches in late pregnancy may be due to the stress of the weight of your uterus on the ligaments that connect it to your spine. Because your uterus dropped a bit, you may be able to breathe and eat more comfortably near the end of your pregnancy. At this time, the lungs of the fetus are likely fully mature and ready to begin breathing. The fetus gains about a half pound every week at the end of pregnancy, for a birth weight of roughly 7 pounds, and is growing longer for a birth length of about 18 to 21 inches. Labor starting on its own around week 40 is a sign that your body is ready to give birth and your baby is ready to be born.