192 Chapter 9 GENERAL DISCUSSION In this final chapter we will use the results of the studies presented in previous chapters to answer the research questions that were raised in the general introduction of this thesis. We will place these results in the current field of science and outline implications for clinical practice as well as the perspectives for future research. Summary A. HSG: a diagnostic procedure with therapeutic effect Although HSG was introduced as a diagnostic test, the fertility enhancing effect of HSG with oil-based contrast has been debated for decades. This therapeutic effect of an HSG with oil-based contrast in couples with unexplained infertility has been established by the clear results of the H2Oil study (2) and its recent replication (1) as well as subsequent systematic reviews and meta-analyses (3, 4). Further research questions regarding the mechanism of action, the efficacy in other groups of infertile couples and the safety were raised. I. Does the fertility enhancing effect of tubal flushing during HSG change over time? We analysed the follow-up data from the H2Oil study up until three years after HSG and calculated consecutive hazard ratios for the chance of the occurrence of an ongoing pregnancy for the group that had received oil-based contrast versus the group that had received water-based contrast. Our results in Chapter 2 show that the hazard ratio for ongoing pregnancy was 1.71 with of oil-based contrast immediately after HSG and reduced to 1 at approximately 2 years after tubal flushing. In other words, the treatment effect of oil-based over water-based contrast during HSG lasts approximately two years. II. What is the effectiveness and cost-effectiveness of the use of oil-based versus water-based contrast medium during HSG in terms of live birth in women who are 39 years or older, women who have a high risk for tubal pathology or who have an ovulation disorder? We have set up the study ‘H2Oil2’ to answer this question in Chapter 3. The study commenced in 2019 and is currently including women in multiple academic and nonacademic hospitals throughout the Netherlands and the United Kingdom. With the results of this study, we hope that a new (inter)national guideline on the use of HSG
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