During pregnancy, the muscles in a woman's intestines will relax. This is actually caused by the hormone progesterone and it will slow the movement of food through the digestive system—hence, constipation. The reason for the constipation during pregnancy due to hormones is because slowing food through the digestive tract will cause two problems: First, the stool will harden and become more difficult to evacuate from your body. Secondly, the stool may compact in the intestines leading to blockages.

Even if the hormones do not substantially affect the intestines, it is still possible for increased risk for constipation during pregnancy due to fetus itself. During the final trimester, the fetus begins to exert pressure upon the lower intestines. This can lead to blockage and ultimately constipation.