Generally, the iron in infant formulas is not believed to cause constipation. But any changes in your baby's formula or feedings could cause changes in number, colour and consistency of his stools. Stool patterns vary normally from child to child and true constipation is infrequent. There is a wide range of normal stool frequency and consistency. Constipation is not the number of bowel movements your baby has each day, but rather it's when those infrequent movements are hard, dry stools that are difficult for your baby to pass. They may come out in hard pellets or cause pain or bleeding. If you think your baby is constipated, call your doctor.