Obstetric outcomes and confounding factors
PTB was classified into two categories—before 37 and 34 weeks—because PTB before 34 weeks required antenatal corticosteroid therapy for fetal maturation.20 pPROM was defined as spontaneous rupture of membranes before 37 weeks. II was identified clinically, which was derived from medical records transcripts. GH was defined as persistently elevated blood pressure (≥140/90 mmHg) after 20 weeks of pregnancy in an otherwise normotensive woman.21
The following items were analyzed as potential confounding factors: maternal age, maternal body mass index (BMI) before pregnancy, parity, maternal smoking status, maternal educational status, and annual household income. Participants were divided into three groups (maternal age <20 years, 20–34 years, and ≥35 years).22,23 BMI before pregnancy was categorized into three groups (<18.5, 18.5–25.0, and <25.0). Parity was categorized into two groups (nulliparous and multiparous). Participants were requested to provide information about their smoking status by choosing one of the following: “kept smoking during pregnancy,” “never smoked,” “quit smoking before pregnancy,” and “quit smoking during early pregnancy.” Participants who chose “kept smoking during pregnancy” were included in the “smoking” category, while the other participants were included in the “non-smoking” category. Educational status was categorized into four groups based on the completed 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 levels (<2,000,000, 2,000,000–5,999,999, 6,000,000–9,999,999, and ≥10,000,000 JPY). These confounding factors were chosen based on clinical importance.24–26