Chapter 2 48 questions on the presence and duration of a menstrual cycle, possible pregnancy, menopause status and contraceptive use status. 2.2. Grouping measurements based on contraceptive status The present study included premenopausal women who had participated at least once but up to five times in assessments between baseline and followup, as is visualized in Figure 2.1A. To exclude postmenopausal women, observations in women aged 55 years or older (n=2391) were excluded. Observations in women who had missing data on the question of contraceptive methods (n=263), or who used hormonal contraceptives other than OC such as a hormonal intrauterine device (IUD; n=373), a hormone implant (n=23), vaginal ring (n=57) or hormone injections (n=34) or other hormonal methods (n=18) were also excluded. Observations during use of hormone implants, vaginal rings and hormone injections were excluded due to low sample sizes, observations in hormonal IUDs were excluded due to unknown date of placement and subsequent unknown quantity of hormone release by the IUD. Observations in women who were pregnant at the time of the assessment (n=195), who gave birth in the last year (n=159) or were breastfeeding (n=7) were also excluded from the analysis. Women who continued OC for more than 90 days, meaning they skipped multiple placebo weeks or “stop weeks”, were also excluded (n=37), as well as naturally cycling women who reported abnormal menstrual cycle durations, defined as current or regular cycles were shorter than 21 days or longer than 35 days (Nederlandse Vereniging voor Obstetrie en Gynaecologie, 2004; n=357). To assess possible selection biases that could stem from excluding women with abnormal cycle durations, an additional sensitivity analysis was conducted on the full sample, including the women with abnormal cycle durations (reported in Supplementary Materials 2.B). Additionally, observations were also excluded if participants were using other hormonal medications (World Health Organization Anatomical Therapeutic Chemical (ATC) code G03, n=22) or using hormone antagonists (ATC code L02, n=2) at the time. The remaining observations were then grouped based on contraceptive status, into either observations during OC use (e.g. OC measurements) or observations during which participants were naturally cycling (NC measurements). Observations in women who did not use any hormonal
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