GSTM1 and gestational diabetes: Information about sample size justification26, 27and a specific description of case and control groups was not provided.27Furthermore, one of the included studies selected the cases and controls regarding the exposure.26Among cohort studies, although Si et al.24showed a positive association between iron supplementation and GDM in women with Hb > 11, some methodological flaws cause concerns regarding the rate of eligible persons and the follow-up baseline being less than 20%.