What causes constipation in pregnancy?

Iron. Some prenatal vitamins have heavy iron load. Insufficient water intake can also contribute. So try a low iron prenatal and drink more water.
Hormones. Progesterone is important to keeping your baby in your womb. It also is responsible for making you feel fatigue and bloated. The bloating is from slowing of your intestines, which can also be called constipation. A high fiber diet, regular exercise, and fluids can counter this condition.