Exposure assessment
Female specific factors in the touch-screen questionnaire included reproductive history and menstrual history, such as the number of live births, use of oral contraceptive pills, history of stillbirth, spontaneous miscarriage or termination, and menarche, age at menopause (last menstrual period), length of menstrual cycle. The number of live births is a useful indicator of reproductive history. To determine the number of live births, participants were asked ‘How many children have you given birth to? (Please include live births only)’ through a touch-screen questionnaire at baseline (2006–2010). Due to small numbers at the highest parities, we pooled women with five or more births. In accordance with previous studies[13,26,27], participants were divided into six groups based on the number of live births: “ 0, 1, 2, 3, 4, ≥5 ”. Our exposure was number of live births, as we wanted to account for number of complete reproductive cycles, including a full pregnancy, a potential breastfeeding period and behavioral changes related to child rearing. Thus, stillbirths, spontaneous miscarriages or terminations were not included in the exposure, and were adjusted in analyses.