44 Chapter 3 1.07, 95% CI 1.00 to 1.14, cumulative live birth rates 74.8% after oil-based contrast, 67.3% after water-based contrast, RR 1.11; 95% CI 1.03 to 1.20) (16). This study additionally demonstrated that HSG with oil-based contrast leads to a significantly shorter time-topregnancy compared to HSG with water-based contrast (10.0 vs 13.7 months; hazard ratio 1.25; 95% CI 10.9 to 1.43). Robust studies investigating the fertility enhancing effect of oil-based contrast during HSG in women who were 39 years of age or older, women with ovulation disorders, and women who have a high risk for tubal pathology are lacking. In the current socio-economic climate, where health care costs are increasing and the importance of evidence-based health care is underlined, the results of previously mentioned studies among couples with unexplained infertility cannot be extrapolated to couples with other types of infertility and therefore separate evaluation is needed. This randomized controlled trial aims to investigate the effectiveness and cost-effectiveness of the use of oil-based versus water-based contrast during HSG, in women with previously unevaluated causes of infertility: women who are 39 years of age or older, women with ovulation disorders, and women who have a high risk for tubal pathology. METHODS This international, multicentre, randomized controlled trial will be performed in university, teaching and non-teaching hospitals in the Netherlands and the United Kingdom. A list of currently participating hospitals is available as Supplementary file 2. The trial has obtained ethical approval by the Institutional Review Board (IRB) of the Amsterdam UMC location Vrije Universiteit (registration number 2018.289), the Research Ethics Committee London Harrow (20/LO/0607), and the board of directors of all participating centres. Participants Women who are scheduled for an HSG as part of their fertility work-up can participate if they meet at least one of the following criteria: (1) women who are 39 years of age or older, (2) women who have an ovulation disorder (ovulation disorders will be defined as less than eight menstrual cycles per year), or (3) have a high risk for tubal pathology (high risk will be defined as a past chlamydia infection, pelvic inflammatory disease, peritonitis, known endometriosis and/or pelvic surgery including tubectomy for ectopic pregnancy). In order to ensure adequate sample sizes in all three groups of participants, women meeting more than one criterion will be included according to the criterion that comes first in ranking. The ranking is based on the expected prevalence of the three subgroups of women within the study population, with the lowest expected prevalence
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