Constipation has always been one of the most common questions/concerns of new parents. Since you mentioned that your baby is getting iron-fortified formula, let me first reassure you that the iron in infant formula is rarely, if ever, responsible for causing constipation, while its benefits are quite clear.
I find that one of the biggest challenges with constipation is the fact that it is quite hard to define. Some babies can poop as infrequently as once a week and still not technically qualify as constipated, while others can go every couple of days but have hard, painful poops and be constipated. It’s worth taking into account the differences between formula fed babies and breastfed babies, since those drinking formula may only poop once every two to three days, which can be in striking contrast to breastfed babies who are known for frequent pooping—sometimes as often as with/after every feeding.
That said, breastfed babies sometimes poop as infrequently as once a week. And when it comes to consistency, you are correct that formula-fed babies tend to have more formed/solid poops than breastfed babies, whose poops may be more liquid than solid. For older babies, any new introductions in a baby’s diet—whether it is formula, water, or baby foods—can all cause very noticeable changes in a baby’s pooping schedule, appearance, and consistency.
The bottom line is that solid/formed poop is not as much of a concern as long as babies are eating and growing well, pooping on a regular schedule, and not in pain or distress when they poop. Babies who are truly constipated are more likely to be uncomfortable, cry, and even eat poorly as a result. Having said that, it’s worth pointing out that some babies strain, grunt, and make everyone in the room aware of their bodily functions even in the complete absence of true constipation.
And finally, to address what can be done about constipation, especially in a young infant, it’s always important to start by discussing your baby’s eating and pooping routines with your pediatrician to make sure that your baby is capable of pooping normally and to determine if your baby is truly constipated. While infants don’t generally need any water at all, your doctor may recommend a very small amount of water (usually no more than 1 to 2 ounces a day) to see if things soften up or require additional attention.