About 18,000 very premature babies are born annually in the United States, with about 5,000 at 22 or 23 weeks.

The study, involving nearly 5,000 babies born between 22 and 27 weeks gestation, found that 22-week-old babies did not survive without medical intervention. In the 78 cases where active treatment was given, 18 survived, and by the time they were young toddlers, seven of those did not have moderate or severe impairments. Six had serious problems such as blindness, deafness or severe cerebral palsy.

Of the 755 born at 23 weeks, treatment was given to 542. About a third of those survived, and about half of the survivors had no significant problems.

Dr. Carl T. D’Angio, director of neonatal clinical research at the University of Rochester Medical Center, who was not involved in the study, said that his center decided several years ago to offer active treatment to 23-week-old babies, and that many sophisticated neonatal units are doing the same. At 22 weeks and five days into pregnancy, Rochester offers corticosteroids to mothers in danger of delivering early, hoping the drugs can have 48 hours to work before delivery at the 23-week mark. But for 22-week-olds, he said, “we don’t have enough to offer the babies to really offer them a reasonable chance of survival.”

As techniques for keeping babies alive improve, parents face wrenching choices that are sometimes based on whether the estimated age is 22 weeks and one day or six days. The study found that hospitals tend to “round up,” with babies closer to 23 weeks more likely to receive treatment.

But the authors and other experts also noted that gestational age is an educated guess, based on women’s recollection of their last menstrual period and estimated fetal size. Other factors, including prenatal care and the fact that girls are often a week more mature than boys, should also influence decisions, experts say.