What Causes Difficulty In Passing Stool In Infants?

i have a 6 month old daughter. she s having problem with her stool. once in every 6 or 7 days her stool gets very hardened and she feels very much tough to release her stool. we used to take her to a pediatrician every 7 days for the same problem. why s this happening to her? what is the remedy for this..