Molly fry with large belly

[Abbix]

My molly gave birth to some fry and all of the darker ones have huge belly’s, they’ve had them since birth and they’re around 3 or 4 weeks old now but they belly is still there and I’m just concerned about what it is and if it’ll go away? They are eating and pooping fine and they are very active and seem happy. My 2 grey ones don’t have this belly though. I’ve attached some photos of the dark ones with the belly’s and the 2 grey ones without the belly’s.