Exposure and other variables
Maternal demographics and lifestyle characteristics included maternal age (≥35 years or no), parity (0 or no), education (university degree and above or no), ART (intrauterine insemination (IUI); IUI with ovulation induction but without in-vitro fertilization (IVF); IVF; IVF with intracytoplasmic sperm injection (ICSI); ovulation induction without IVF and vaginal insemination), pre-pregnancy body mass index (BMI) categories (underweight, normal, overweight/obese).
Maternal pregnancy characteristics and complications included gestational age at delivery, delivery mode (vaginal delivery, either spontaneous or by vacuum extraction or forceps, and cesarean section, either planned or by emergency), multiple pregnancy, gestational diabetes mellitus (GDM), pregnancy induced hypertension (PIH), hypothyroidism, preterm birth (<37 week), postpartum hemorrhage (>500 mL after vaginal delivery, blood loss >1000 mL after cesarean delivery), premature rupture of membranes, ischemic placental diseases (composite of preeclampsia, intrauterine growth retardation (IUGR), placental abruption and stillbirth), placenta previa, abruptio placentae and postpartum transfusion.
Newborn characteristics included fetal sex, birthweight, small for gestational age (SGA) ≤10th, large for gestational age (LGA) ≥90th according to Chinese sex- and gestational age-specific birth weight standards[17], macrosomia (>4000 g), low birthweight (<2500 g), very low birthweight (<2500 g), sentinel congenital anomalies (atrial septal defect, ventricular septal defect, esophageal fistula, hypospadias), respiratory distress syndrome and hyperbilirubinemia (>12 mg/dL).