I read with great interest the interesting study examining preterm birth rate in the United States using 2 separate methods of gestational age (GA) assignment: GA calculated by the last menstrual period and the obstetric estimate of GA. They report that when an obstetric estimate of GA was used, 15% fewer live births in the United States were classified as preterm with a lower rate of preterm birth, 9.7% vs 11.5% ( P < .001). However, the description of preterm birth in this paper is different from the universally accepted definition from the World Health Organization, which defines preterm birth as a live birth occurring at less than 37 completed weeks of gestation.

In the Methods section of this paper, preterm birth was defined as any live birth at 36 or fewer completed weeks by the last menstrual period or an obstetric estimate of gestational age. This also differs from the American Academy of Pediatrics and American College of Obstetricians and Gynecologists, which define premature birth as any live birth occurring through the end of the last day of the 37th week (259th day) following the onset of the last menstrual period. When using numerical descriptions in Figure 1, however, the authors describe their preterm group of infants appropriately as (infants < 37 weeks). These 2 variable definitions of preterm birth in this study cause confusion in the interpretation of the data and results.

It is important to clarify that completed weeks of gestation are defined by the number of 7 day intervals after the first day of the last menstrual period. The 37th week of gestation (which ranges from 36 0/7 to 36 6/7) is completed when the last day of week 37 (that is 36 6/7) is completed.

Using the author’s definition of preterm birth as live birth at 36 or fewer completed weeks (that is, ≤ 36 0/7) is incorrect because of the chance of exclusion of babies born after 36 but before 37 completed weeks’ gestation (ie, those 36 0/7 to 36 6/7). This definition would account for inaccurate data collection with a potential for underestimation of preterm births. The discrepancy in the interpretation of the definition of preterm birth is noteworthy because of the possible impact it can have on the assessment of the global burden of prematurity.

This letter calls for the standardization of internationally accepted definitions of preterm birth to enhance our understanding of the current issue and help improve our efforts at prevention.