(HealthDay News) — Differing methods used to calculate gestational age affect the accuracy of the reported rate of preterm births in the United States, according to research published in the August issue of the American Journal of Obstetrics & Gynecology.

Elaine L. Duryea, MD, of the University of Texas Southwestern Medical Center in Dallas, and colleagues used data from U.S. birth certificates for 2012 to determine the rate of preterm birth (defined as 36 or fewer completed weeks of gestation). The computed rates of preterm birth were compared for different methods of calculating gestational age: one using the obstetric estimate specified in the revised birth certificate and the other using the estimate based on the last menstrual period alone.

The researchers found that the rate of preterm birth was significantly lower when the obstetric estimate was used to calculate gestational age (9.7%) rather than the estimate based on the last menstrual period alone (11.5%). When the obstetric estimate was used, neonates identified as preterm were more likely to qualify as low birth weight (54 vs. 42%; P<0.001) and have known morbidities associated with preterm birth, such as need for assisted ventilation and surfactant use.

“Because the majority of European and other industrialized nations utilize the obstetric estimate for reporting purposes, a change in U.S. reporting practices would serve to facilitate a more meaningful international comparison as well as more accurately estimate the rate of preterm birth,” the authors write.

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