GSTM1 and gestational diabetes: For multiparous pregnant women, BC was of the highest effect on Hb (β: − 0.85, 95% CI: − 1.65, − 0.04, g/L, per IQR increase) after adjusting for age, gestational age, GWG, ethnicity, educational level, occupation, gestational hypertensive disorders, gestational diabetes, GDP, population density, and season of blood examination, which was in line with the general knowledge that BC was the most harmful component of PM2.5.