Nash Keen was born prematurely weighing just 10 ounces — less than a can of soda — and doctors doubted he would survive. He recently had his first birthday.

Although most studies measuring premature-baby outcomes start at 22 and 23 weeks — reporting U.S. survival rates between 10 and 20 percent at that gestation — University of Iowa Health Care is among the few hospital systems that take on babies at 21 weeks.