Our DS (29 months) has been withholding poop since June. He had a couple episodes in April and May but started consistently holding in June. So far, he has not been constipated but he was having sore poops from what we know now are likely multiple food intolerances/allergies. This has been an extremely difficult situation for us.
We saw a pediatrician early on who recommended Miralax but we decided to first try natural solutions. Miralax is scary! There is a yahoo group that was started by parents who had children who had terrible reactions to Miralax. I highly recommend joining this list because there are many knowledgeable people on it who can help with more natural alternatives. What we did to clear the backlog of poop is bake pears (peel, dice & bake in the oven at 375 for 45 mins or so) and add a little prune juice (just an ounce or so to start.) We also gave a little bit of barlean's organic flax oil, which you can order online (don't overdo it, start with just 1/4 tsp a day.) We also ordered something called Fruit-eze, which we put in sunbutter and jelly sandwiches as the jelly (don't over do this either, a tsp or so to start.) It is all natural - prunes, dates, etc. We got the backlog of poop cleaned out this way.
However, it didn't fix the problem.
We started a very detailed food journal - logged every morsel and symptom. We also started to 'help' him poop by picking him up when he went to cross his legs, hold him in a squatting position, hug & comfort him, which relaxed him enough to go. This kept him relatively caught up (he usually was pooping out the food from 2-3 days prior.) The problem was, his diaper area seemed to always be sensitive and not really improving. At first we thought it was all because of the behavior (holding, smearing, etc.) and that did exacerbate it but now we know it was primarily because he was still eating foods he was sensitive to. We are now egg, dairy, soy, corn, wheat and gluten free. All the while, we eliminated a few at a time (in retrospect, this was a dumb thing to do, we should have cut it all out immediately. It was just hard because our diets were so laden with all the foods we were cutting that we were struggling with what to eat!) It wasn't until we eliminated wheat and gluten just recently that we saw what appears to be a big turn-around. He still needs to be held to feel comfortable enough to poop (he asks us to hold him and he still resists a while before going) but he is at least letting it out and his bottom is healing with no more obvious redness or rash from the poop. He is eating foods we suspected (peas, carrots, etc) with no issue. He even seems to maybe have picked up some weight already and tonight he pooped out yesterday's
food. I'm not sure we've figured it all out but I see enough signs to be highly optimistic that we are finally on the right track.
When he was younger, we always thought maybe he might have food sensitivities but the rash was minor and intermittent and every time we eliminated something we suspected from our detailed food journal, it didn't seem to help and we couldn't pinpoint it. I don't know what mental block kept me from thinking 'WHEAT'. I think it was mostly because he would eat wheat on some days and things would seem okay. The delayed reaction plus the withholding behavior made it VERY hard to figure out. We should have just cut it all out.
Also, to start, when he started holding consistently in June, it was after a visit from our parents and we thought it might have been stress related - he has always been highly sensitive. Retrospectively, we realize he had really
loaded up on all the major allergens during big family meal times! For example, at one dinner, he ate a ton of lasagna, filled with dairy AND wheat plus the acidic tomato, which just made it all worse.
My recommendation is to try simple nature methods first, prune juice, baked fruit (pears are awesome for pooping) and flax oil while immediately starting a total elimination diet (if you're still nursing, you'll have to totally eliminate too.) Try holding her when she has to go. If those methods don't work, try both the yahoo groups for Miralax and encopresis (let me know if you can't find them) because they have a ton of natural suggestions for constipation.
As for allergies, your ped can do a blood test or you can also look into these:http://www.yorktest.com/
offers a finger prick test you can do at home for food intolerances.http://www.enterolab.com/
offers a stool test for milk/soy/yeast/egg/gluten (and a gene test for celiacs)https://www.alcat.com
offers a blood test (a draw is required)