Dear Editors,

We read with great interest the study by Melo & Araújo ¹1. Melo GC, Araújo KCGM. COVID-19 infection in pregnant women, preterm delivery, birth weight, and vertical transmission: a systematic review and meta-analysis. Cad Saúde Pública 2020; 36:e00087320.. The authors aimed to analyze whether the COVID-19 infection in pregnant women is related to premature birth and birth weight, and to investigate the possibility of vertical transmission. The authors should be acclaimed for establishing a timely and informative systematic review of COVID-19 in this critical condition, however, we would like to draw their attention to a certain aspect.

The authors should extract the hospital’s name and the periods of recruitment because some pregnant women may have been included in multiple publications, as admission dates overlap with reports from the same hospital. The cases in reference 30 (Yang et al. ²2. Yang P, Wang X, Liu P, Wei C, He B, Zheng J, et al. Clinical characteristics and risk assessment of newborns born to mothers with COVID-19. J Clin Virol 2020; 127:104356.) should be considered duplicates because the case series from reference 9 (Chen et al. ³3. Chen H, Guo J, Wang C, Luo F, Yu X, Zhang W, et al. Clinical characteristics and intrauterine vertical transmission potential of COVID-19 infection in nine pregnant women: a retrospective review of medical records. Lancet 2020; 395:809-15.) was reported by the same hospital with the overlapping periods of recruitment. At least one pregnant woman or one neonate had identical characteristics (e.g., maternal age, gestational age at delivery, mode of delivery, gender, Apgar score, birth weight).

The inclusion of duplicates may introduce bias into a systematic review and affect the comprehension of the disease and its epidemiology. Thus, we suggest that the authors review the hospital information and the periods of recruitment. If the periods of recruitment overlap and a hospital had published their cases more than once, only the study with the biggest data should be included to minimize the possibility of double counting ⁴4. Cai J, Tang M, Gao Y, Zhang H, Yang Y, Zhang D, et al. Cesarean section or vaginal delivery to prevent possible vertical transmission from a pregnant mother confirmed with COVID-19 to a neonate: a systematic review. Front Med (Lausanne) 2021; 8:634949..

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