If the child is old enough to eat semi solid food, then puréed prunes, peaches, apricots, or just about any other fruit can help to relieve the constipation. Also, make sure the baby is well hydrated. Avoid bananas and rice products though, because this does the opposite.
If the baby still drinks milk only, then a warm bath, a tummy massage, or putting the baby on his back and help them do "bicycle legs"
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Great answer. I had two children that had this problem, and had to be sure they got their fruit every day. We had to switch off of similac with iron. Iron can be very constipating. Mixing fruit with their cereal also helped.