By Gene Emery
Girls, babies that weigh more and babies that do not have a twin all survived premature birth better, as did babies whose mothers were treated with steroids to hasten the development of the lungs, the researchers reported in The New England Journal of Medicine.
These factors also lowered the risk of neurodevelopmental problems such as blindness, hearing loss, thinking problems and cerebral palsy.
"Using the five factors in combination really gives you a better idea of how children are going to do, rather than singling out a single factor," Dr. Rosemary Higgins of the National Institute of Child Health and Human Development (NICHD), who worked on the study, said.
The findings are designed to help doctors and patients decide whether to give aggressive treatment or only comfort care to such children.
Usually, doctors treat an extremely premature infant primarily on the basis of its gestational age, but they cannot always tell with certainty when a child was conceived.
The researchers, led by Jon Tyson of the University of Texas Medical School at Houston, studied 4 446 infants born 22 to 25 weeks after conception. A full-term pregnancy lasts 40 weeks.
They said 49 percent of the infants in the study died and 21 percent survived without a disability based on tests done when they were about two years old.
"We found that about half of the infants survived and that about half of those who survived had neurodevelopmental impairments. This is a very, very high risk group of babies," Tyson said in a statement.
The NICHD, which funded the study, has used the data to create a Web site that outlines the odds of healthy survival in specific cases. A calculator here is designed to guide doctors and families.
For example, the site estimates that a 23-week-old girl weighing 800 grams who received steroid treatment and was born alone has a 36 percent chance of survival without a neurodevelopmental impairment. At 22 weeks, the likelihood drops to just 12 percent.
"Sometimes gestational age can be off by a week or two, depending on dating or ultrasound," said Higgins. "As you can see from the study, a week makes a big difference."
So does gender. A boy born at 23 weeks under those circumstances has a 22 percent chance of surviving without impairment, although the researchers stressed that those numbers are just estimates based on the new data.
"There clearly appears to be a biologic difference that gives girls a week advantage over the boys for the same gestational age. It's a difference that's unexplained," Higgins said.