50 Chapter 3 DISCUSSION Increasing female age is one of the main causes of infertility in the twenty-first century, with >50% of the women undergoing IVF being over 35 years of age (29, 30). Anovulation and tubal pathology are also important causes of infertility. As a consequence, the results of this H2Oil2 study are estimated to be applicable to more than 50% of infertile women seen in fertility clinics. This multicentre randomized controlled trial will generate insight in the potential fertility enhancing effect of tubal flushing using oil-based contrast during HSG in infertile women who are 39 years or older, women with ovulatory disease, and women with a high risk for tubal pathology. The generated evidence can guide clinicians and policy makers to decide which subgroups of women will benefit from an HSG with oil-based contrast as a therapeutic intervention, whether the intervention is cost-effective and if the risk of adverse events is acceptable. Strengths and limitations The proposed study is specifically designed to detect a difference in ongoing pregnancies leading to live births for three important subgroups of infertile women. Most randomized clinical trials regarding the fertility enhancing effect of oil-based contrast during HSG have been previously performed in couples with unexplained infertility and/or mild male factor. While several trials have also included women with other types of infertility (31-34), none were able to show a significant positive effect of tubal flushing with oilbased contrast mostly due to a low sample size, and the results of previous trials among women with unexplained infertility cannot simply be extrapolated to all women facing infertility (14). Another strength of this study is its multinational character. As it involves academic and non-academic (teaching and non-teaching) participating centres from the Netherlands and the United Kingdom, the results will be applicable to different countries with different hospital settings. A limitation of this study is the potential diversity in treatments between the various participating centres. The participating centres will treat patients according to their local protocol. Although these protocols are based on national guidelines, there is variety between the national guidelines of the participating countries (4, 35). As this study evaluates two variants of standard care which are already applied in current practice, we chose this pragmatic approach to generate evidence that is applicable to the majority of the treating centres. This approach will lead to a difference in management between participating centres, possibly influencing the chance of conception. Randomization is therefore stratified per inclusion group and per inclusion site, to prevent uneven distribution among the two randomization groups.
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