157 Migraine attack incidence in relation to ovulation 7 Sensitivity and validation analyses To evaluate the reliability of the calculated ovulation day, we determined the mean difference between the calculated ovulation day (the 14th day before the start of menses), and the day of positive LH-surge urine test as gold standard in the WHAT-hormone subgroup. This difference was on average 0.28 days (95% CI: -0.17-0.73). A complete case analysis was conducted by including the 100 participants initially excluded due to chronic migraine, resulting in a final cohort of 654 participants. Within this group, the ovulatory window was also not associated with with migraine attack incidence compared to all other cycle days(OR: 0.96, 95% CI: 0.90-1.02, p=0.16) (Table e-4), which is similar to the results obtained in the primary analysis (OR: 0.95, 95% CI: 0.88-1.02, p=0.13). DISCUSSION In this study we found no increased incidence of migraine attacks (with or without aura) during the ovulatory phase in women. Interestingly, we observed a relatively higher incidence of migraine attacks during the follicular phase compared to the luteal phase, possibly due to the residual effects of perimenstrual estrogen decline. Previous studies examining the relationship between migraine and the menstrual cycle faced limitations due to inaccurate cycle timing measurements. Ovulation is most reliably identified using ovulation tests that detect the luteinizing hormone (LH) surge. Another important finding of our study is that, in women with regular menstrual cycles, ovulation can be reasonably estimated as the 14th day before menses. Visual representations of migraine attack frequencies in relation to the menstrual cycle are commonly standardized to a 28-day cycle, anchored to the first day of menstruation. 5, 12, 13 While this approach allows for general comparisons, it may complicate the interpretation of migraine attack incidence during the follicular phase and ovulation. This is due to the averaging out of intraindividual variations in cycle length, which may result in differences in phase-specific timing between individuals and makes it challenging to precisely study events such as ovulation. However, we were able to estimate the ovulatory window in women with a regular menstrual
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