Background. Recently three large-scale epidemiological registry-based
Scandinavian studies examined the association between use of hormonal
contraception and the risk of developing depression or use of
antidepressants. They reached surprisingly divergent results.
Objectives: The aim of this study was to explain why these three recent
studies from Denmark and Sweden could achieve quite different results,
interpretations, and conclusions. Search strategi and selection. The
three existing large scale Scandinavian studies examining associations
between exposure to different types of hormonal contraception and risk
of depression or use of antidepressants were examined according to
chosen design, exclusion criteria, and included confounders.
Methodological choices were considered, and the validity of these
methodological choices tested. Main results. First, the assumption that
differences between studies are due to residual confounding is proven
unlikely, already because confounder control beyond age, year and
education rarely change estimates materially. More likely basic
differences in chosen study groups, exclusions from the study groups,
exposure definitions, chosen reference populations, and interpretation
of the results seem to explain the differences between the studies.
Conclusion. The detailed review of the three Scandinavian studies
reveals methodological choices as the main explanation for their
different findings. Residual confounding was found unlikely to explain
the divergent results, while ideological circumstances might have a main
responsibility for the different chosen methods and for the
interpretation of the results. Funding. None.