Declines in birth weight and fetal growth independent of gestational length.

OBJECTIVE: To estimate whether the decrease in birth weight of term singletons in the United States and elsewhere over the past decade, despite trends in common maternal characteristics expected to contribute toward an increase, is attributable to the simultaneous decrease in gestational length.
METHODS: Using data from Intermountain Healthcare, where a successful initiative reduced the number of early-term (37-38 weeks) elective deliveries, we examined trends in birth weight, being small for gestational age (SGA), and being large for gestational age (LGA) among 219,694 singleton neonates born between July 2000 and December 2008 at 37-41 weeks of gestation.
RESULTS: Over the 8.5 years, births through scheduled deliveries at 37-38 weeks decreased (9.7-4.4%), but overall scheduled deliveries increased (29-34%) and mean gestational age at birth (39.1 weeks) did not change. Mean birth weight (3,410-3,383 g) and LGA status (9.0-7.4%) both decreased, whereas SGA increased (7.5-8.2%). In multivariable analyses adjusting for maternal and newborn characteristics, birth weight decreased (-36 g; 95% confidence interval [CI] -31 to -42), especially among neonates born at 37-38 weeks of gestation (-40 g; 95% CI -30 to -49) or among those with medical indications for urgent deliveries (-48 g; 95% CI -34 to -63). Odds of being LGA decreased (0.77; 95% CI 0.73-0.82) and odds of being SGA increased (1.12; 95% CI 1.06-1.19).
CONCLUSION: Even in a population in which gestational length did not change, birth weight and fetal growth declined. Decrease not only in gestational length but also in fetal growth is likely to be contributing to the widely observed recent decrease in birth weight. LEVEL OF EVIDENCE: I.