May 19, 2018 - Epidemiology and Statistics Sorbonne Paris Cité, Paris. Descartes .... Promoting Arab and Israeli cooperation: peace building through health ...
Correspondence
Clarification of INTERGROWTH-21st newborn birthweight standards We are writing regarding José Villar and colleagues’ INTERGROWTH-21st Project on international standards for newborn birthweight (Sept 6, 2014, p 857).1 We are assessing these standards, along with the project’s estimated fetal weight standards,2 against French birthweight data. In doing so, we found it confusing whether the centile references in Villar and colleagues’ study refer to completed weeks of gestation (ie, 36 weeks meaning all infants from 36 weeks plus 0 days, to 36 weeks plus 6 days) or exact weeks of gestation (ie, 36 weeks meaning infants born at 36 weeks plus 0 days only). In the Lancet Article, the authors state that “the data are reported according to completed weeks of gestation (WHO ICD-10) as smoothed centiles”. As would be expected for values www.thelancet.com Vol 391 May 19, 2018
representing completed weeks, the figure in this letter places the centile points between the gestational ages. However, on the INTERGROWTH-21st Project website,3,4 these same values are presented as referring to exact gestational ages (ie, the value for 36 weeks in the INTERGROWTH-21st study is 36 weeks plus 0 days, according to the study website). The possibility that these standards refer to exact, as opposed to completed, weeks is supported by the INTERGROWTH-21st Project‘s estimated fetal weight study.2 The authors show that the estim ated fetal weight centiles, which are based on exact weeks,5 are similar to birthweight centiles at term. Interpreting these standards as referring to exact, as opposed to completed, weeks results in a shift of the centile thresholds towards higher values. The magnitude of this difference is shown, based on French national data on singleton births from 2016 (figure). This difference could have important consequences for research and clinical care. Given the international convention of presenting newborn birthweight centiles by completed weeks and the description provided by Villar and colleagues1 supporting this convention, it is probable that the newborn standards will be interpreted as referring to completed weeks. We assumed that the centiles applied to com pleted weeks in our study of European data, which were presented at an international conference.7 The authors of a Canadian study8 have also told us that they used this interpretation. If the standards refer to exact weeks, results of these and possibly other studies using the newborn standards will need to be corrected. Clarifying this question is necessary to ensure that the standards of the INTERGROWTH-21st Project are used appropriately by researchers to reach valid scientific conclusions, and by health-care providers in their care of pregnant women and newborn babies. We declare no competing interests.
100 90
Percentile >97th 90–97th 50–89th 10–49th 3–10th
