Focused on nearly 4,200 extremely premature infants, the study shows babies born between 22 and 25 weeks gestation were more likely to survive, if

    They were born at a later gestational age
    They were female
    Their mothers were given antenatal corticosteroids shortly before giving birth
    They were born single, rather than part of a multiple birth
    They had higher birth weights

    Traditionally, doctors use gestational age, the number of weeks from fertilization to birth, to determine whether a preemie should receive invasive care.

    "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 John Langer, a co-author of the study, who works in Maryland as a statistician for the North Carolina-based Research Triangle Institute.

    Extremely premature babies are those born after 22 to 25 weeks in the womb. They often weigh just 1.5 pounds (0.68 kg) and measure 10 or 11 inches (25.4-28 cm). A full term is about 40 weeks.

    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.

    Preemies face toughness of survival and often are placed on breathing machines or given other special help.

    "The researchers' findings provide important information that physicians and family members can consult to help them make the most informed treatment decisions possible, " said Dr. Duane Alexander, director of the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD).