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].