Hospitalization of early preterm, late preterm, and term infants during the first year of life by gestational age.

OBJECTIVES The goal of this study was to describe hospitalizations of infants during the first year of life according to week of gestational age (GA). We hypothesized that odds of any hospitalization would generally decrease with increasing GA, with late preterm infants experiencing additional increased risk of specific hospitalizations, such as hyperbilirubinemia. METHODS Birth certificates for > 6.6 million infants born in California hospitals between 1993 and 2005 and surviving to discharge were linked to hospital discharge records during the first year of life. Odds of any hospitalization and any hospitalization for specific diagnoses during the first year of life were determined for infants 23 to 44 weeks' GA. Further analysis determined odds of any hospitalization within 14, 30, and 90 days of birth discharge, and observed odds were compared with expected odds obtained through quadratic modeling. RESULTS Odds of any hospitalization within the first year of life decreased with advancing GA, but observed odds of any hospitalization exceeded expected odds for 35-, 36-, and 37-week GA infants for all time periods after discharge. Odds of any hospitalization for hyperbilirubinemia were greatest for infants 33 to 38 weeks' GA (peak odds ratio at 36 weeks' GA: 2.86 [95% confidence interval: 2.73-3.00]), and a relative peak in odds of any hospitalization for specific infections was observed among infants 33 to 36 weeks' GA. CONCLUSIONS Odds of any hospitalization during the first year of life exceeded expected odds of hospitalization for 35-, 36-, and 37-week GA infants. GAs at risk overlapped with, but were not identical to, GAs identified as late preterm infants.