Obstetric outcomes and confounding factors
LBW was classified into two categories: LBW < 2,500 g and LBW < 1,500 g.1 SGA was defined as a birth weight below 1.5 standard deviations, corrected for gestational age and sex, according to “New Japanese neonatal anthropometric charts for gestational age at birth”.8 Macrosomia was defined as neonatal birth weight >4,000 g.12
The following parameters were considered as potential confounding factors: maternal age, body mass index (BMI) before pregnancy, parity, maternal smoking status, maternal alcohol consumption status, maternal educational status, annual household income, PTB before 37 weeks, and gestational BWG. Maternal age was categorized into three age groups: <20 years, 20–29 years, and ≥30 years based on a previous study that showed that maternal age was related to certain obstetric outcomes, such as PTB, LBW, and SGA.26 BMI before pregnancy was categorized into three groups (<18.5 kg/m2, 18.5–25.0 kg/m2, and >25.0 kg/m2).27,28Parity was categorized into two groups (nulliparous and multiparous). Maternal participants were requested to provide information about their smoking status by choosing one of the following: “Yes, I still smoke”; “Never”; “Previously did but quit before recognizing current pregnancy”; and “Previously did but quit after learning of current pregnancy.” The maternal participants who chose “Yes, I still smoke” comprised the smoking category, whereas the other participants comprised the non-smoking category. Maternal participants were requested to provide information about their alcohol consumption status by choosing one of the following: “never drank,” “quit drinking before pregnancy,” “quit drinking during early pregnancy,” and “kept drinking during pregnancy.”29 The maternal participants who chose “kept drinking during pregnancy” comprised the drinking category, while the other participants comprised the non-drinking category. The educational status of the mother was categorized into four groups on the basis of the number of years of education (junior high school, <10 years; high school, 10–12 years; professional school or university, 13–16 years; and graduate school, ≥17 years). Annual household income was categorized into four groups (<2,000,000 JPY, 2,000,000–5,999,999 JPY, 6,000,000–9,999,999 JPY, and ≥10,000,000 JPY). PTB was defined as the birth before 37 weeks of gestation, the record of which was collected from Dr-0m data. The data for gestational BWG were obtained from Dr-0m data, which included information about body weight before pregnancy (kg) and body weight just before delivery (kg). Gestational BWG was defined as body weight just before delivery minus body weight before pregnancy (kg). We defined appropriate BWG as less than 12 kg and excessive BWG as more than 12 kg, according to the criteria aimed at appropriate birth weight, described by Ministry of Health, Labour and Welfare, Japan.27,28 These confounding factors were chosen on the basis of the clinical importance.5,13,14