Chapter 2 62 most used form of OC in the Netherlands, with 90.6% of combined OC users using this formulation (data from 2018; GIP databank, 2021). Hence, it is expected that the majority of the OC-users in our study used this form. In the current study we could only rely on population-based estimates of what forms of OC are mostly used, but future studies should take OC formulation into account whenever possible. Another limitation is related to the exclusion of women with abnormal cycle duration, which meant that our results could not be generalized to women who report abnormally long or short cycles. Since abnormal cycle lengths can be a reason to start with OC, there was a risk of sample bias, since this meant that we mostly excluded women with abnormal cycle during NC measurements. We addressed this by conducting a sensitivity analysis on the larger sample including women with abnormal cycle lengths. The results of this sensitivity analysis show similar results to analysis in the original sample, which only included women with regular cycle lengths (see Supplementary Materials 2.B). In summary, our study found consistent associations between OC use and insomnia symptoms, although effect sizes of these estimates were small. Furthermore, we found no overall associations between OC use and depression outcomes, both with regards to symptom severity and diagnosis prevalence. However, post-hoc analyses showed that the within- and between-person estimates indicated significant associations between depression and OC use. This indicates that, although OC overall was not associated with depression outcomes, some individuals might experience OC-associated depression symptoms. The effect sizes of the found associations were small, and future research should be conducted to assess whether effects of OC on depression risk are robust and clinically relevant. Nonetheless, our findings on this topic show novel results on within-person associations between OC and depression diagnoses obtained through clinical interviews, which underscore the importance of accounting for individual differences in OC research. Although our study was limited in some aspects of OC use, such as duration of use, our findings raise new questions on which women might be susceptible to possible adverse mood symptoms during OC use, and whether insomnia symptoms might be affected by OC. Future research should focus on studying underlying mechanisms between OC, insomnia and depression.
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