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.