2.4 Covariates
Four types of factors were considered to be covariates. Mother’s sociodemographic characteristics included maternal age (less than 25, 25-30, and more than 30 years old), occupation (housewife/others), and education (less than or equal to 9, 10-12, and more than 12 years)[15]. Pregnancy-related characteristics consisted of parity (primiparous/multiparous), delivery mode (natural birth/cesarean), pregnancy hypertension (yes/no), delivery complications (yes/no), work during pregnancy (yes/no), and tobacco exposure during pregnancy (yes/no)[15, 16]. Pregnancy complications included gestational hypertension (HDP), amniotic fluid embolism, uterine rupture, abnormal umbilical cord, and so on. Work during pregnancy (no) meant pregnant women didn’t work during the whole pregnancy. Both active and passive smoking were classified as maternal tobacco exposure due to the low number of active smokers, passive smoking is defined as more than 30 minutes of tobacco exposure per week. In addition, we also selected infant gender (male/female) and family income (less than 6000 yuan/more than or equal to 6000 yuan)[15].