Baby girls of both races had the strongest advantage when born weighing less than 1,000 grams, about 2 pounds or as much as a quart of milk, Morse said. Girls had nearly twice the odds of surviving as baby boys did, and black infants also had a slight survival advantage over whites, the research shows. Overall, black baby girls were twice as likely to survive compared with white baby boys, 1.8 times more likely to survive than black boys and 1.3 times more likely to live than white baby girls.
"When you're talking about survival, that's very significant," Morse said. "We have known in general that females tend to have better survival rates than males and blacks better than whites. But quantifying that and finding if there was a statistical significance had yet to be done."
Morse and other researchers from the UF Maternal Child Health Education and Research and Data Center also analyzed the infants' developmental ages and weights at birth, combining these data with race and gender to specify the odds of survival for babies born in each demographic.
Nationwide, nearly a half million babies are born prematurely each year, according to the National Center for Health Statistics. Only about 1 percent of all babies born weigh less than 2 pounds, and one of the first questions parents of these infants ask is if their child will live, said Morse, who as a neonatologist works with families every day. Having accurate data can help families and doctors make better decisions at a time when choices can be hard to make, he said.
"I'm trying to get as much information as I can before the baby is born to give the parents a realistic expectation of survival," he said. "Not all babies are the same, especially with regard to survival at this early gestational age. There are differences based on race and gender, so we can't group all these babies together and say survival at less than 1,000 grams is X-percent."
UF researchers studied vital statistics from 5,076 babies born in Florida between 1996 and 2000 and weighing less than 1,000 grams. The influence of gender and race on babies' survival rates was more noticeable the smaller the infants were, the research shows. The higher the weights and developmental ages were at birth, the more survival rates increased for all babies.
About 1,500 babies included in the study were extremely premature, born when their mothers were less than 24 weeks pregnant. On average, these babies had a less than 27 percent chance of survival. Because their organs have not had as much time to develop, these tiny babies are at the highest risk for disabling health problems, and doctors and families often struggle to decide what life-saving measures should be taken, if any, Morse said.
"This is the highest-risk population of babies and there is a lot of controversy, especially at the lower gestational ages, of how much should we really do for these babies, how aggressively should we treat them, especially around 23 to 24 weeks," he said.
The researchers do not know what measures were taken to save the lives of each of the babies included in the study, which Morse describes as the only limitation of the research. Many families decide just to hold their babies as they pass away, while others adopt a wait-and-see approach or request all measures be taken to save the preemie's life.
"It's very hard to make rules as to which babies should be resuscitated," said Herman A. Hein, a professor of pediatrics at the University of Iowa. "Each case should be addressed with individuals. These babies do have a tremendous risk, but it's interesting when you talk to families and ask them if they regret their decision. I have yet to find one that says yes."
Hein says the findings of the UF study do not surprise him. Prior research has shown that black women tend to have more premature babies than women of other races, possibly because their babies mature a little earlier and faster, he said.
There isn't conclusive evidence yet to explain why girls and black infants have better chances of survival, Morse said. But female preemies' lungs tend to be more developed at birth, which could be part of the explanation, Morse said.
For Morse, the next big question isn't why these babies survive but what happens to them when they do. He now plans to study what happens to extremely low-birth weight children, who are more prone to health problems, as they age.
"Survival is not everything," he said. "It's a first step. Probably a bigger question to answer is quality of life. That's the next step."