Many women are surprised to learn that even very early in pregnancy, it's possible to feel (and look) several months pregnant. This is due to abdominal bloating, which begins as early as the first few days after conception and can last well into the first trimester. Though weight gain and true maternity clothes are many months away, newly pregnant moms can feel as though their pelvis is quite full, and it's not uncommon to see the beginnings of a tiny baby bump---with the caveat that the bump isn't actually because of the baby just yet.
Uterine Swelling
During week four of pregnancy, a fertilized egg is implanting in the uterine wall. Until the placenta develops, the blood vessels of the endometrium, or spongy lining of the uterus, will provide nourishment for the developing embryo. For this reason, though the endometrium proliferates and thickens each month as a normal part of the menstrual cycle (and is subsequently shed each month during the menstrual flow if no fertilization takes place), it thickens even more than usual in the early weeks of pregnancy. Sending vastly increased amounts of blood to the uterus forces the heart to work hard, and heart rate increases in order to keep blood flow high. The uterus swells in response to its increased blood supply and rapidly thickening lining, which causes it to take up more room in the pelvic cavity, leading to a bloated, congested feeling in the lower abdomen, explains Dr. Miriam Stoppard, in her book "Conception, Pregnancy and Birth."
Gas
Heidi Murkoff and Sharon Mazel, in their classic book "What to Expect When You're Expecting," note the important role of early pregnancy hormones. These hormones dictate changes in a woman's body that prepare it to support an embryo (and eventually a fetus) throughout pregnancy. The hormone relaxin, which allows the pelvis to stretch so that a baby can pass through during delivery, is produced from very early in the pregnancy. A side effect of this hormone is that it relaxes the muscles of the digestive tract as well, meaning that food stays in the intestine longer than usual. Naturally occurring bacteria in the gut are therefore given more opportunity than they usually have to digest fiber and other matter that is indigestible to humans, and the product of bacterial digestion is gas. For this reason, pregnant women have more gas than those who are not pregnant, and this can contribute to bloat.
Constipation
As the gut slows down, food reaches the terminus of the intestine---the rectum---less frequently than it normally would, leading to less regular bowel movements. Further, the job of the large intestine is to extract nutrition and water from food, and if food sits in the intestine for a long period of time, both of these processes become more efficient. This will eventually be of great benefit to the fetus---a slow intestine is better at extracting all the nutritional content from each of mom's meals---but an uncomfortable side effect is that the additional water extracted leaves stools drier than usual. These dry stools are particularly slow at completing their journey to the rectum, and built up fecal matter in the lower gastrointestinal tract is yet another cause of maternal bloat early in pregnancy.
References
- "Conception, Pregnancy and Birth"; Miriam Stoppard, M.D.; 2008
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
