First, let's address the iron and formula question. Some people will (or have) told you that the iron is the problem and that switching to a low-iron formula will help. It makes sense, because when adults take iron supplements, we get constipated as a side effect. But there was a study done concerning the formula question in babies. Half got low iron, half got regular--not knowing which was which--and the rate of constipation was the same in both groups. For most kids that does not make a difference, and the iron is very important for growing babies.

The constipation can be formula-related, though. Allergies to some of the proteins in the formula could cause the discomfort. Sometimes switching to another brand will help (the milk-based formulas are not all the same). Sometimes switching to soy will help. All other things being equal, soy makes stools firmer--but for kids with an isolated allergy to milk protein, the constipation may dramatically clear up on soy.

Still, about 20 percent of kids with a cow's milk protein allergy will also have a soy allergy at that age, so if it doesn't work you still will not know if this is formula related. Most allergies to milk and soy are outgrown as infants or toddlers

I like your pediatrician's suggestion of switching to Nutramigen. It is a hydrolysate formula, which means the proteins have already been chopped into building blocks so they are easier to digest and pretty impossible to be allergic to. If Nutramigen does not work you will know that formula is not the issue.

Carnation Good Start is another good possibility. It is an example of milk-based formulas not all being the same. The milk proteins in Good Start are partially hydrolyzed, so it may be easier for some kids to digest, but it will not do the trick for a full-blown cow's milk protein allergy.

If the formula is not the issue, there is a long list of rare causes that could be causing the constipation. Hirschprung's disease is probably the most important. This is where there are not enough nerve endings in the end of the colon to push stool out effectively.