Birth by cesarean section, or C-section, is common among women but not so common among cats, although sometimes a difficult birth does require a vet to anesthetize a mother and deliver kittens by incision. This does not harm the kittens, but it does mean the kittens will be temporarily motherless, since the mother has to recover from anesthesia.
The surgery tends to throw her maternal hormones off balance, which means she may be unwilling to nurse the kittens, pulling away from them as they try to grab hold and suckle. Usually she comes around and allows them to nurse, but in some cases she never does, meaning the kittens have to be hand-fed or fostered out to another
nursing cat.
