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.
Some doctors said they were startled to see that certain factors equated to an extra week in the womb.
"That's the thing that catches my attention," said Dr. David Rubenstein, director of the neonatal intensive care unit at New York City's Columbia University Medical Center.
The researchers also found that in cases where boys and girls had equal chances of survival, girls were less likely than boys to receive intensive care. It's not clear why, but Langer said heavier babies tend to get intensive care more often, and boys tend to be heavier.
Some parents of preemies said they're not sure what they would have done with this new information had they had it at the time of birth.
Amy Schatz of New York gave birth to a 24-week-old boy in 2004. Before the birth, her doctor tried to prepare her for the worst by telling her boys don't survive as well as girls, she said.
"I was devastated. It really frightened me," said Schatz, 45. Her son, Noah, is now healthy and developing normally.
Sean and Jolene Tuley of Mount Juliet, Tenn., dealt with greater tragedy. They were expecting twins when, in January, the placenta of one child _ a boy named Ayden _ detached from Jolene's uterus.
With no time to give the mother steroids, doctors performed an emergency Caesarean section and delivered the children at 23½ weeks.
A doctor told them the twins faced dangers and impairments _ especially Ayden, who had a collapsed lung and serious brain bleeding. "Do we continue treatment, or let him go?" recalled Sean Tuley.
The Tuleys instructed the doctor to keep providing care for both. Clara lived, and doctors think she may be able to go home from the hospital this week. But Ayden died after nine days.
It's important that parents have all the information they can when facing a decision about care in a situation like that, said Jolene Tuley, 33.
But she also echoed Schatz: It's not clear what parents can do about factors like whether the preemie is a boy or a girl or if the child had steroids. "It's not something you can control," she said.