Data collection
We used the dataset released in June 2016 (dataset: jecs-ag-20160424) for this study. This dataset consisted of three types of information: (1) self-reported questionnaire obtained around the first trimester, including medical background (e.g. endometriosis, uterine myoma, adenomyosis, method of conception, maternal height, and maternal weight before pregnancy), or food frequency questionnaires (FFQs); (2) self-reported questionnaire collected during the second/third trimester, including socioeconomic status such as maternal education or household income; and (3) obstetrics outcome that was retrieved from the medical records of each subject’s institution. In the present analysis, we used the FFQ data that were completed during the first trimester, and diet intake was assessed 1 year before pregnancy. This tool was used in the JECS and has been used as a self-administered diet questionnaire in previous Japanese epidemiological studies.20 In the present study, we excluded cases with insufficient data and multiple pregnancies.