Objective: To determine in a geographically defined population 1 year survival of infants with a birthweight of less than 1500 g or gestational age less than 32 weeks, and to establish the effect of postnatal age on predicted survival.
Design: Cohort analysis of 72,427 births to Welsh residents in 1993-94. Deaths were identified using the All Wales Perinatal Survey, a population-based surveillance of mortality between 20 weeks of gestation and 1 year of age.
Main outcome measures: Birthweight- and gestation-specific infant mortality, and the effect of postnatal age, gender, and multiple pregnancy on predicted survival.
Results: In normally formed infants 1 year survival at 24-25 weeks gestation was 35%, compared to 75% at 27-28 weeks, and 95% at 30-31 weeks. In infants with a birthweight of 500-699 g 1 year survival was 18% compared to 70% at 800-999 g, and 97% at 1300-1499 g. The chances of survival improved markedly with increasing postanatal age; at 24-25 weeks gestation it was 35% at birth, 50% at 12 h. 66% at 7 days and 78% at 4 weeks. Infant mortality was higher in males, but multiple pregnancy had no effect.
Conclusions: Up-to-date birthweight- and gestation-specific survival rates are essential for predicting the outcome of a newborn infant. The rapid change in the chances of survival with increasing postnatal age emphasises especially the importance of revising the prediction as the infant gets older, particularly during the first few days of life.