Persistence of underweight status among late preterm infants.

OBJECTIVE To determine the association of late preterm gestation (34-36 weeks' gestation) with underweight status in infancy. DESIGN Retrospective cohort study. SETTING Thirty-one primary care sites within a hospital-owned network, from January 1, 2007, through June 30, 2009. PARTICIPANTS Seven thousand eight hundred sixty-six infants with gestational ages ranging from 34 to 42 weeks, followed up through the first 18 months of life. Analytic sample consisted of 7624 infants examined at 6 months of age; 7132, at 1 year; and 6957, at 18 months. MAIN EXPOSURE Late preterm (34-36 weeks), early term (37-38 weeks), or full-term (39-42 weeks) gestation. MAIN OUTCOME MEASURES Weight-for-age z score of 2 or less at 6, 12, and 18 months. RESULTS Compared with full-term gestation, late preterm gestation was associated with increased adjusted odds ratios (AORs) of weight-for-age z score of 2 or less at 6 months (AOR, 3.48 [95% CI, 2.17-5.72]) and 12 months (2.22 [1.07-4.61]). At 18 months, this association was not significant (AOR, 1.62 [95% CI, 0.69-3.84]). After exclusion of infants who were small for their gestational age, late prematurity was associated with underweight status when defined as a decline from birth weight of more than the 10th percentile to a weight-for-age z score of 2 or less at 6 months (AOR, 3.35 [95% CI, 1.76-6.38]) and 12 months (2.72 [1.02-7.27]) but not 18 months (1.88 [0.64-5.55]). CONCLUSIONS There is an association between late prematurity and underweight status in the first year of life. Further research is needed to determine the effect of this growth pattern on developmental outcomes and to optimize nutritional management.