My son completely stopped pooping when we were potty training. The Dr. told us that they feel the need to have control over something during this difficult time. I was so worried - we actually switched Dr's over this. He would not poop for a month, and we had to give him enemas. He would hold it in and hold it in until it was too painful for him to go at all.

Finally, we switched Dr's, and she said that this is completely normal. She gave us a medicine that came in a powder that we could put in his juice at bedtime. Then he had no choice but to poop. We were able to put less and less powder into his juice until we were finally able to stop a few months later.