Aptamil is a good formula for preventing constipation I would say, judging by what people say on forums and my own experience. I've used it with both my kiddies and have always had 2 runny poos a day, regular as clockwork (sorry if TMI!). Altogether though it's worth saying that all formulas are equal in terms of nutrition so whatever one you end up choosing or your little one gets on with best, your LO will thrive X