Validation of obstetric estimate using early ultrasound: 2007 California birth certificates

Obstetric estimate (OE) of gestational age, recently added to the standard US birth certificate, has not been validated. Using early ultrasound-based gestational age (prior to 20 weeks gestation) as the criterion standard, we estimated the prevalence of preterm delivery and the sensitivity and positive predictive value (PPV) of gestational age estimates based on OE.

We analyzed 165 148 singleton livebirth records (38% of California livebirths during the study period) with linked early ultrasound information from a statewide California prenatal screening programme. OE of gestational age estimates was obtained from birth certificates.

Prevalence of preterm delivery (<37 weeks gestation) was higher based on early ultrasound (8.1%) compared with preterm delivery based on OE (7.1%). Sensitivity for preterm birth when using OE for gestational age was 74.9% (95% confidence interval [CI] [74.1, 75.6]), and PPV was 85.1% (95% CI [84.4, 85.7]). Incongruence, defined as a ≥ 14-day difference between early-ultrasound-derived gestational age and OE, was 3.4%.

OE reported on the birth certificate may underestimate the prevalence of preterm delivery, particularly among women of non-Hispanic non-white race and ethnicity and women with lower educational attainment, public insurance at time of delivery, and missing prepregnancy BMI. Additional validation studies in other samples of births are needed.