It can't be iron though, as it's not soluble in milk. In fact, that's the reason why, in the 3rd trimester of pregnancy, babies stock iron for the first 4-6 mon

It can't be iron though, as it's not soluble in milk. In fact, that's the reason why, in the 3rd trimester of pregnancy, babies stock iron for the first 4-6 months of (postpartum) life, basically, for all the way up until eating solids, and why pre-term babies have to be supplemented with iron because they didn't have a chance to complete stocking.