Doctors have made substantial progress in saving the earliest premature babies, with fewer illnesses and disabilities among them, according to a report issued Tuesday by an agency of the National Institutes of Health that looked at two decades of developments in the field.

Babies born between 22 and 28 weeks of gestation and who weighed 400 to 1,500 grams (14.1 to 52.9 ounces) have benefited from new practices instituted between 1993 and 2012, the period of the study, said Rosemary Higgins, program scientist for the neonatal research network at the National Institute of Child Health and Human Development, who was senior author on the paper. Normal gestation is 40 weeks, and the vast majority of full-term babies weigh between 5.5 and 10 pounds at birth.

"Extremely pre-term babies born before the 28th week are now surviving in greater numbers, and their outcomes are better when you look at the illnesses they have" in neo-natal intensive care units, Higgins said in an interview.

Between 1993 and 2012, the study found a "significant increase in survival" of infants born at 23, 24, 25 and 27 weeks, according to results published Tuesday in the Journal of the American Medical Association.

Infants born at 25 to 28 weeks also showed major improvement in survival without significant disease or disability, according to the paper, though few surviving infants born sooner than that were able to avoid those illnesses. The study looked at more than 34,000 pre-term infants born at academic medical centers between 1993 and 2012.

The overall rate of pre-term birth peaked at 12.8 percent in 2006, then eased to 11.39 percent by 2013.

The prevalence of late onset sepsis, a complication of infection, declined between 2005 and 2012, for example, despite little progress before then. The rate of severe bleeding in the head also dropped. But one condition, bronchopulmonary dysplasia, a lung condition, saw a sharp increase during the period studied, possibly because of improvements in care that allowed more infants at risk for the illness to survive, Higgins said.

Higgins pointed to a number of advances in the care of severely pre-term babies. Increased use of surfactant has offered better protection for the newborns' lungs, and steroids given to mothers in the hours or days before a pre-term birth promote development of lungs, which normally don't mature until 34 to 36 weeks of gestation, she said.

Fewer pre-term newborns are now placed on ventilators to help them breathe because doctors can rely more on continuous positive airway pressure machines, which blow air into the lungs to help infants breath, but are gentler on them.

Mothers are now routinely screened for strep infections and given antibiotics if it is found, Higgins said. And providers now emphasize feeding severely pre-term newborns with breast milk instead of formula, even if they are only able to deliver a few drops through a feeding tube, because of the protections it provides the child, she said.

The study did not examine whether the new techniques have affected length of stay in hospitals, which averaged 93 days for newborns who survived, or costs.