It's a common problem of pregnancy, caused by changes that take place in the body. An increase in the hormone progesterone causes the smooth muscles of the gastrointestinal tract to relax.
Food is propelled through the digestive tract more slowly.
Also, pressure from the enlarging uterus and the use of iron supplements can slow the movement of food through the intestines. To prevent constipation, select a wide variety of fibre-rich foods, drink plenty of water and get regular exercise. The fibre from food creates bulkier, softer stools that pass through the body more quickly and easily. Water is also excreted with the fibre so it is important to drink plenty of water to prevent dehydration and hard dry stools. Exercise helps maintain muscle tone and increases the efficiency of body processes.