The researchers said gestational age has long been the factor most commonly used for such predictions. But now the National Institutes of Health Neonatal Research Network has added, birth weight, gender, whether the baby is a twin and whether the mother was given antenatal steroid mediation to aid the baby's lung development.

The study involved a statistical analysis of the health records of 4,446 children born between 22 and 25 weeks of pregnancy and weighing between one and two pounds at birth said Dr. Mark Mercurio of Yale University School of Medicine, a member of the consortium. Researchers examined which factors, aside from gestational age, influenced the outcomes for extremely low-birth-weight infants.

They found an infant's chances of survival without disability were enhanced if they were of older gestational age, their mothers had been given antenatal steroids, they were female, they were singletons rather than part of a multiple birth and they had higher birth weight.

The research is reported in the April 17 issue of the New England Journal of Medicine.