Completed Weeks of Gestation and Risk of Respiratory Syncytial Virus in Late Preterm Infants

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I read with great interest the prospective observational study,1 the respiratory syncytial virus (RSV) preterm (32–36 completed weeks of gestation) risk estimation measure for RSV hospitalization (RSVH) in Ireland with the objective to identify RSVH population-specific risk factors for preterm infants. RSV prophylaxis was offered in this study to preterm infants with gestational age (GA) of 32 weeks and 0 days to 36 weeks and 6 days. Reported to be the first study to include late preterm (LPT) infants beyond 35 weeks of GA, it helps characterize the epidemiology and independent risk factors for community-acquired RSVH among 32–36 weeks of GA infants. This study highlighted GA-specific RSVH risk and found that birth from 36 weeks to 36 + 6 days mitigated RSVH risk (relative risk: 0.58; 95% confidence interval: 0.34–0.99) and that the risk factors were similar to the cohort including preterm babies with GA 32 weeks and 0 days to 35 weeks and 6 days.
However, it is important to note that using the terminology “32–36 completed weeks of gestation” to represent the infants “32 weeks and 0 days to 36 weeks and 6 days” GA included in this study is inaccurate. This is because 36 weeks are completed at 36 weeks and 0 days and not at 36 weeks and 6 days as reported by the authors in this article.1 The concept of “completed weeks” of gestation2 needs clarification in that the thirty-sixth week of gestation is completed when the last day of “week 36” (which ranges from 35 weeks and 0 days to 35 weeks and 6 days) is completed, that is, the thirty-sixth week of gestation is completed at the end of 35 weeks and 6 days. As a matter of fact, 36 weeks and 6 days of GA represent the last day of week 37 of gestation. Thus, the use of the terminology “32–36 completed weeks” of gestation in the paper is misleading as it implies that the study includes babies born with GA 32 weeks and 0 days to 35 weeks and 6 days; when in fact, it includes the subset of LPT babies born between 36 weeks and 0 days and 36 weeks and 6 days.
Also, it is important to clarify that LPT infants include babies born between 34 completed weeks (34 weeks and 0 days or day 239) and less than 37 completed weeks (36 weeks and 6 days or day 259) of gestation as defined by the National Institute of Child Health and Human Development of the National Institutes of Health,3 and the World Health Organization categorizes preterm babies born “32 to <37 weeks” GA as “moderate to late preterm.”4 It is imperative that all providers use standardized definitions for “completed weeks” of gestation and for LPT infants in future studies evaluating outcomes of RSV prophylaxis in preterm infants.
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