Dr. Greene's Answer
Congratulations on number 3! On average, parents are the most exhausted when their third child is 4-6 weeks old (no matter how many kids they end up having). I guess that means you have new energy to look forward to before very long.
When kids in the first month or two spit up, it is much more likely to be a mechanical issue than a formula intolerance. The valve at the top of the stomach may be too loose (so it can’t hold things in), the valve at the bottom of the stomach may be too tight (so the stomach gets too full), or big air bubbles may take up too much room.
Smaller, more frequent feedings are more likely to help than switching formulas. If “projectile splat” of spit-up shoots across the room, it’s important to consider the condition called “pyloric stenosis”. Here, the too-tight valve at the bottom of the stomach needs to be treated.
As for the constipation, the most likely cause is liquid not getting in (and staying in) — again more likely than a formula intolerance. Different formulas do produce different consistencies of stools, and switching formulas may help.
Switching brands is more likely to help than changing the amount of iron. Most kids do not have softer stools on low-iron formulas, but a few iron-sensitive kids do. Low iron is not a good long-term choice, but it is safe for a while, just to get beyond this problem. Soy formulas, by the way, tend to produce firmer stools.