The enigma of 36 completed weeks of gestation: 36 0/7 or 36 6/7?

Abstract

In 2008, an American College of Obstetricians and Gynecologists (ACOG) committee opinion1 defined late-preterm (LPT) infants as infants born between 34 0/7 weeks and 36 6/7 weeks of gestation. However, this is often mistakenly believed by authors to represent 34–36 completed weeks of gestation, as in this article.2 As a matter of fact, 34–36 completed weeks of gestation includes babies born between 34 0/7 and 36 0/7 weeks gestational age. This is because 36 weeks is completed the day after 35 6/7, which is 36 0/7. With ongoing evidence that LPT babies have poor outcomes when compared to full-term infants, it is imperative that we use standardized terminology for LPT infants when planning future studies. In 2005, the National Institute of Child Health and Human Development (NICHD) of the National Institutes of Health (NIH) defined LPT birth (LPTB) as births between 34 completed weeks (34 0/7 weeks, or day 239) and less than 37 completed weeks (36 6/7 weeks, or day 259) of gestation.3 This therefore represents a 3-week period as opposed to the definition used in this paper, wherein the authors defined LPTB as birth between 34 and 36 completed weeks gestation. The definition used in this paper is incorrect, as it excludes babies between 36 0/7 and 36 6/7 gestational age, and this subset should be included in the late preterm4 cohort. The authors seem to have misinterpreted the ACOG committee opinion (34 0/7 to 36 6/7 weeks of gestation) by defining LPTB as birth between 34 and 36 completed weeks of gestation. Their conclusion that 45% of respondents correctly defined LPTB as birth between 34–36 completed weeks gestation (which is an incorrect definition) is thus inaccurate. Simply stated,4 36 weeks of gestation are completed at 36 0/7, and therefore, 36 completed weeks is numerically represented as 36 0/7 and not 36 6/7. I would like to urge all providers to have uniformity in utilization of standardized definitions when using the terminology “completed weeks of gestation.” The use of universally accepted terminology is important for consistency in literature and future studies evaluating outcomes of LPT infants.