Heres what I found on the hormone (progesterone) and costipation, the hormones that help cause constipation are in the 2nd paragraph:
Pregnant women may experience constipation, which can be very uncomfortable. Constipation is a common problem during pregnancy. Constipation refers only to bowel movements that are hard in consistency or painful to eliminate. Infrequent bowel movements (e.g., less than once daily) are not unusual. During pregnancy the muscles in your intestines become more relaxed causing constipation. In the last months of pregnancy, the growing baby puts pressure on the lower intestines
and this may also cause constipation. Medication can induce constipation such as supplemental iron and calcium. Other contributing factors include hormonal changes that cause the intestinal tract to slow down, prolonged immobilization (bed rest), little exercise, inadequate intake of fiber and fluids, irregular eating habits, and possibly stress.
During pregnancy, the body produces more female hormones than normal. It is the job of these hormones to make sure that the pregnancy develops normally. But they also automatically slow down the woman's intestinal movements. One of these hormones, called progesterone, acts by relaxing muscle. The wall (outside) of the bowel is made of muscle and relaxes, and so is less able to move food and bodily waste along. The pregnancy hormone progesterone slows down intestinal function, and the longer the stool sits in the large intestine, the harder and drier it gets. While they have many benefits, prenatal vitamins, particularly those containing iron, are very constipating. Calcium-based antacids (like TUMS
What cugirl found is great information. You see, it's more of a combination of hormones in your body that make constipation a problem. Along with progesterone, the hormone relaxin also plays a factor in constipation, along with water retention and hands/feet swelling. HTH