A cesarean section, or c-section, is a surgical procedure in which incisions are made through a woman's abdomen and uterus to deliver her baby. The procedure is performed in the United States on nearly one of every four babies delivered--more than 900,000 babies each year. C-sections are performed whenever abnormal conditions complicate labor and vaginal delivery, threatening the life or health of the mother or the baby.

The most common reason that a cesarean section is performed (in 35% of all cases, according to the U.S. Public Health Service) is that the woman has had a previous c-section. The "once a cesarean, always a cesarean" rule originated back when the incision in the uterus was made vertically; the resulting scar was weak and had a risk of rupturing during later deliveries. Today, the incision is almost always made horizontally across the lower end of the uterus (this...