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