Odds have improved that many extremely premature U.S. infants will survive 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 5 of them survived without major complications.

The research highlights the importance of just a few more weeks in the womb: Among preemies 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 preemies 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.

The researchers reviewed hospital records from 1993 through 2012 for nearly 35,000 extremely premature babies born at 26 academic centers participating in a National Institutes of Health research network. It's a large number of infants, but the findings may not reflect outcomes at other types of hospitals.

Included infants were born at 22 weeks to 28 weeks of pregnancy, weighing about 14 ounces to just over 3 pounds. Normal pregnancies last 39 to 40 weeks.

Information was available on survival until hospital discharge, which for tiny preemies can be months after birth.

Among all preemies born in 1993, 70 percent survived until hospital discharge, versus 79 percent in 2012; rates varied by gestational age.

Survival without major complications increased by about 2 percent each year for babies born at 25 weeks to 28 weeks, and by 2012 more than half the oldest preemies who survived until hospital discharge had no major complications.

Serious complications in tiny preemies can include infection, brain bleeding, a retina disease that can cause blindness, severe intestinal disease and lung problems.

While rates for many conditions declined for many infants, there was an increase for some preemies in a chronic lung illness called bronchopulmonary dysplasia, which can develop in babies whose lungs are immature and who need oxygen treatment or breathing machines.

A JAMA editorial says the research documents important progress, yet there is no obvious "breakthrough therapy" emerging in coming years to help tiny preemies survive and thrive.

"An additional commitment must be made to further improvements in the decades to come," Dr. Roger Soll, a newborn infants specialist at the University of Vermont, wrote in the editorial.