CHICAGO — More extremely premature US infants are surviving without major problems, although prospects remain poor for the smallest and youngest, born nearly four months too soon, a government-funded study found.
The findings suggest that the age of viability barely budged over 20 years: In 1993, just 6 percent of babies in the study born at 22 weeks survived long enough to leave the hospital, versus 9 percent in 2012. Of 1,550 infants born at 22 weeks during the 20-year study just 99 survived until at least hospital discharge, and only five of them survived without major complications.
The research highlights the importance of a few more weeks in the womb: Among babies born at 27 weeks, survival without major problems climbed from 29 percent in 1993 to 47 percent in 2012.
Improvements are likely due to changes in medical practices, including more Cesarean section births, which may be gentler for the most fragile infants, better infection control procedures, newer methods for helping babies breathe without using ventilators, and giving women steroids before childbirth to boost lung growth, the researchers said.
‘‘Our findings are cautiously optimistic. Progress is being made,’’ said Dr. Barbara Stoll, the lead author and head of pediatrics at Emory University’s medical school in Atlanta.
The study was published in Tuesday’s Journal of the American Medical Association.