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.

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On The Net:

New England Journal: http://nejm.org