Measuring Gestational Age in Vital Statistics Data: Transitioning to the Obstetric Estimate

Beginning with the 2014 data year, NCHS is transitioning to a new standard for estimating the gestational age of a newborn. The new measure, the obstetric estimate of gestation at delivery (OE), replaces the measure based on the date of the last normal menses (LMP). This transition is being made because of increasing evidence of the greater validity of the OE compared with the LMP-based measure.

A new NCHS report describes the relationship between the two measures. Agreement between the two measures is shown for 2013. Comparisons between the two measures for single gestational weeks and selected gestational age categories for 2013, and trends in the two measures for 2007–2013 by gestational category, focusing on preterm births, are shown for the United States and by race and Hispanic origin and state.

Data are derived from U.S. birth certificates for 2007–2013 for 100% of reported resident births.

Key Findings from the Report:

  • Estimates of pregnancy length were the same for the OE-and LMP-based measures for 62.1% of all births, and within 1 week for 83.4% in 2013.
  • The mean OE-based gestational age for all 2013 births was 38.5 weeks, lower than the LMP-based average of 38.7.
  • Births were less likely to be classified as preterm using the OE (9.62%) than with the LMP (11.39%). The 2013 OE preterm rate was lower than the LMP rate for 49 states and the District of Columbia.
  • The OE-based percentage of full-term deliveries was higher than the LMP-based percentage; levels of late-term and postterm deliveries were lower. Preterm birth rates declined for both measures from 2007 through 2013 (8% compared with 10%).
  • The OE-based 2013 preterm infant mortality rate was 19% higher than the LMP rate.
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