ATLANTA (AP) _ Doctors now have a better way of helping parents make an agonizing decision _ whether to take heroic steps to save a very premature baby.

The number of weeks in the womb has generally been the chief factor. But a new study shows others are important, too _ including whether the infant is a girl and whether the child gets lung-maturing steroids shortly before birth.

Those extra factors can count as much as an extra week of pregnancy.

The new information could change how doctors and parents decide what kind of care to provide to tiny, fragile premature infants, said John Langer, a co-author of the study being published this week in the New England Journal of Medicine.

Besides being a girl and getting the steroids, an extra 3½ ounces or so of weight and being a single birth also helped as much as an extra week of pregnancy, the study found.

"For the first time, parents and their doctors will have the best available information on which to base one of the most difficult and time-sensitive decisions they are ever likely to face," said Langer, who works in Maryland as a statistician for the North Carolina-based Research Triangle Institute.

The research focused on extremely premature babies, those born after 22 to 25 weeks in the womb. A full term is about 40 weeks.

Extremely premature babies face some of the longest odds of survival and often are placed on breathing machines or given other special help. They often weigh just 1½ pounds and measure 10 or 11 inches _ not much longer than an average adult's hand.

These births present parents with a terrifying choice _ whether to take extreme measures to save the child, possibly destined for a life of severe disability, or stop treatment and allow the child to die.

The new study focused on nearly 4,200 extremely premature infants born at hospitals across the country.

Half died within two years after birth. About 12 percent survived but had significant impairments like blindness, deafness or cerebral palsy. About the same number had even more severe physical or mental disabilities.

The researchers put together an online tool that invites people to type in an infant's birth weight, gender and other data and predicts survival odds based on the study's results.

Gestational age _ the number of weeks from fertilization to birth _ is closely connected to chances of survival. In the study, of babies with a gestational age of 22 weeks, 95 percent died. At 23 weeks, about three-quarters died. At 24 weeks, less than half died, and at 25 weeks, only about a quarter died.

Premature babies born at 24 weeks or older are routinely given intensive care, but smaller babies are handled case by case, said Dr. Judy Aschner, chief of neonatology at Vanderbilt University's children's hospital in Nashville, Tenn.

But gestational age is an imperfect measurement, often based on a mother's memory of her last period before a pregnancy began, and may be off by a week or two.