People say tummy tuck, and really they are asking about a variety of operations designed to reduce fat, remove extra skin, improve old scars, or generally rearrange the lay of the skin and fat of the lower abdomen. The most bothersome aspect of some weight gain in early adulthood is often just a simple excess of the lower skin. In the most significant cases the excess develops into an overhanging area of tissue.
I often get asked if that tissue can get removed at the time of a C-section. The correct term for an operation that removes an area of over-hanging skin is panniculectomy. And it's usually done with a wedge-shaped incision. In the days before prophylactic antibiotics (everyone gets at least a dose or two of antibiotics during their surgeries) infections of the incision at the time of C-section was much more common, and we didn’t want to risk any increased risk with extra surgery.
It is also true that much of the abdominal fat is going to resolve naturally and that many physicians feel this would be a very unnecessary surgery. It is also true that the distention of the uterus has produced skin stretching that will resolve in the postpartum period, without surgery, although it may not resolve completely. It is also true that there are FDA approved laser stretch mark treatments that will produce a better cosmetic result in the end than a panniculectomy in some cases. There are other, individual considerations, let alone the fact that in pregnancy we have extra blood vessels that are more likely to bleed than if you wait until after pregnancy to even consider such surgeries.