GSTM1 and gestational diabetes: When all data were combined, high Hb (≥ 130 g/L) was significantly associated with odds of maternal mortality (OR (95% CI) 2.01 (1.12–3.61)), gestational diabetes (OR (95% CI) 1.71 (1.19–2.46)), and pre-eclampsia (OR (95% CI) 1.34 (1.16–1.56)).