My breastfed baby got v constipated when we introduced formula and it lasted a month, where she'd only poo once every 5-7 days. We got some lactulose from the GP at one point. She's now back to ebf and pooing multiple times a day again. I think if I tried formula again I would try a different brand.
I saw the GP with my eldest when she went 11 days without pooing and was told it’s perfectly normal in breastfed babies! Baby 2 seems to be more regular but usually every other day at the moment. I do find the car seat helps though!
I’ve ebf since birth and my lg went 8 days without pooping. Apparently it’s very normal for bf babies as they absorb most of it. My hv said they can go up to two weeks without a poop before they’d be concerned