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The Iron if contained in your prenatal vitamins can cause constipation.
During pregnancy even otherwise the body delayed gastric motility to allow for better absorption of food for the growing needs for the growing baby. So constipation can result from the delayed gastric motility also.
Have a good diet with about 3 servings of vegetables and 3 servings of fruits per day. Drink plenty of water to combat constipation.
Take care.