Usually babies get constipation from too much iron in the formula. My kids couldn't have the gold stuff cos it gave them horrible constipation - DD1 was the WORST of all the kids though, and she was on goats milk formula - much gentler on their tummies and easier for them to digest. So DD2 is on that now too.

Anyhoo - I'd try the Step 2 - just because apparently it doesn't have as much iron in it as the Step 1 cos they are eating solids after 6 months and getting iron from other sources.

HTH.