288 association with (adverse) pregnancy outcomes. We are however of the opinion that due to the (much) larger size of our control group versus the adenomyosis group, this effect will have been sufficiently minimized however. We purposefully selected a broad control group in order to as far as possible reflect obstetric outcomes in the general population versus those with certain adenomyosis (as opposed to for example controls without adenomyosis at hysterectomy, as this group would represent a selected population with an indication for hysterectomy in the first place). Despite the obvious benefits to using large and anonymized national databases, their use did introduce several constraints to the amount of patient information available. First, as visible in Figure 1, a large proportion of women with adenomyosis could not be linked to pregnancy outcomes. This is most likely due to limits regarding the years of available data and missing patient information. It is plausible that many women did experience pregnancies, but fell outside the study period. One could also hypothesize that as adenomyosis is linked to infertility (84,88,281), a large number of women with adenomyosis may not have been able to become pregnant in the first place, although this is purely speculative. Additionally, pathological reports gave little to no information on the type of adenomyosis, making it difficult to draw conclusions regarding the effect of adenomyosis severity on obstetric outcomes. Likewise, in the Perined registry, certain potential confounding factors such as BMI and smoking were not (well) reported. We attempted to correct for these confounders by using the proxy of low- socio-economic background. Other potentially relevant patient characteristics such as miscarriages and mode of conception were also not well reported. Conclusions: This is the largest study to assess adverse obstetric outcomes in women with adenomyosis diagnosis based on histopathology. Our results confirm that women with histologically proven adenomyosis exhibit a higher prevalence of adverse obstetric outcomes, particularly for hypertensive disorders, failure to progress in labour and placental issues. Future prospective studies should investigate the extent to which non-invasive methods of adenomyosis
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