MRI Assessment:
Two investigators (MvdW and CR) independently reviewed all pelvic MRIs for signs of adenomyosis blinded to the final histopathological diagnosis. Adenomyosis was suspected when one or more of the following features was present: (irregular) JZ >12mm, presence of myometrial high signal intensity (HSI) foci and/or asymmetric enlarged uterus (other than due to presence of leiomyoma’s). Measurements were done using Spectra IDS7 version 21.1 (Linköping, Sweden). Table S1 shows an overview and definition of the parameters that were measured. Consensus was reached if there was a difference of <2mm. If discrepancies existed between the assessments of the two investigators, expertise was sought from a pelvic radiologist (J.N.). The researchers independently concluded whether an MRI adenomyosis diagnosis was suspected, after which the pathology report was consulted to review the conclusive histopathological diagnosis. The influence of uterine contractions on JZ measurements was minimised by confirming (maximal) JZ thickness in more than one imaging plane. In the case of bad quality MRIs, or extremely abnormal uteri affecting the ability for assessment, only those MRI parameters that could be reliably measured were assessed.