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