Obstetric outcomes and confounding factors
PTB was classified into two categories: delivery before 37 weeks and before 34 weeks (severe PTB). LBW was also categorised into two categories: <2500 g and <1500 g (severe LBW). In this study, hypertensive disorder of pregnancy (HDP) was defined as a new onset of hypertension (≥140/90 mmHg) after the 20th gestational week for previously normotensive women. Confounding factors were determined by clinical importance, i.e. those believed to be related to PTB and dietary habits. The following items were used as confounding factors: maternal age, maternal smoking status, maternal education, household income, maternal body mass index (BMI) before pregnancy, myoma uteri, adenomyosis, and assisted reproductive technology (ART) pregnancy. Maternal age was categorised into six groups: ≤19, 20–24, 25–29, 30–34, 35–39, and ≥40 years. Maternal prepregnancy BMI was calculated by dividing the height (m) by the square of the body weight (kg). We categorised participants into three BMI groups as follows: <18.5, 18.5–25.0, and ≥25.0 kg/m2. T1 data provided information on their smoking status during the first trimester: “never smoked,” “quit smoking before pregnancy,” “quit smoking during early pregnancy,” and “kept smoking during pregnancy.” Women who “kept smoking during pregnancy” were considered smokers (smoking category); otherwise, they were considered as non-smokers (non-smoking).Maternal education was categorised into four groups as 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 categorised into four levels (<2,000,000; 2,000,000–5,999,999; 6,000,000–9,999,999; and ≥10,000,000 JPY). Maternal participants were also asked to answer the question: “Have you ever been diagnosed with endometriosis (or other pre-pregnancy gynaecological condition, i.e. adenomyosis, uterine myoma) at a medical institution?” Maternal participants who answered “Yes” were classified as having uterine anomaly (or other pre-pregnancy gynaecological condition). The manner of conception was categorised as natural pregnancy or ART pregnancy, with ART defined as conception after in vitro fertilisation and/or intracytoplasmic sperm injection, or cryopreserved, frozen, or blastocyst embryo transfers.