A previous report found a larger effect for preterm birth (10), mostly due to the inclusion of births at 22–23 weeks of gestation in that report. Recent declines in the U.S. infant mortality rate and percentage of preterm births, and the use of the obstetric estimate to measure gestational age in the current report (compared with gestational age based on the last menstrual period used in the previous report), may have also contributed to the difference in findings between the two reports.