Decreased Term and Post-term Birthweight in the United States: Impact of Labor Induction

Abstract

Changes in birthweight (BW) and gestational age (GA) at delivery are important measures that affect maternal and neonatal outcomes. Increases in both mean BW and rates of macrosomia occurred from the early 1970s to the early 1990s in the United States and several Western nations, despite a continuing rise in the rate of preterm birth. However, in recent years, the rate of macrosomia has fallen in the United States. Simultaneously, the rate of labor induction has doubled, due in large part to an increase in the rate of elective induction since the 1990s. These investigators observed the recent trend toward decreased BW in the United States and speculated that it might not be solely due to increases in preterm (<37 weeks) birth, but might be due in part to the increased rate of labor induction and cesarean delivery. This ecological study investigated the association between the rising rates of labor induction and temporal changes in BW and GA at delivery by examining temporal trends in these parameters among US non-Hispanic white singleton live births ≥37 weeks of gestation. Data on live birth cohorts for the study period (1992 through 2003) were obtained from US vital statistics. The independent association between the change in rates of labor induction and changes in mean BW, mean GA at delivery, and rates of macrosomia was estimated using ecological linear regression analysis. During the 12-year study period, mean BW decreased by 37 g, mean GA at delivery declined by 3 days, and there was a 25% relative reduction in the rate of macrosomia. Simultaneously, rates of labor induction nearly doubled, rising from 14% to 27%. Linear regression analysis showed a significant association between the increased rate of induction and reduced mean BW (r = −0.54 [95% confidence interval (CI), −0.71 to −0.29), reduced mean GA at delivery (r = −0.44 [95% CI, −0.65 to −0.17]), and lower macrosomia rates (r = −0.55 [95% CI, −0.74 to −0.32]). These findings suggest that increasing use of elective induction may be responsible for the observed recent decline in both GA and BW.