Many prenatal vitamins are high in iron, which can cause your elimination to slow down (as can separate iron supplements used to help treat anemia). Some prenatal vitamins contain a stool softener to minimize the problem. If yours doesn't, talk to your doctor about switching brands or trying a vitamin that has DHA (a nutrient that helps baby's brain development -- and is also a natural stool softener).
But constipation is extremely common during pregnancy and is likely caused by a number of factors in addition to your prenatal vitamin. For one thing, increased levels of the hormone progesterone flooding your body can also cause your digestive tract to become sluggish. And as your baby grows, he can crowd other organs that affect digestion, making your bathroom breaks less regular.
To feel better, make sure you're getting enough to drink (about eight glasses of water or other fluids a day), which helps speed along digestion. And be sure to eat plenty of high-fiber foods like whole grains, veggies, and fruits (peaches, pears, and plums are especially great). If the problem doesn't go away or starts becoming painful, your doctor may add a fiber supplement to your vitamin regimen.