26 Chapter 2 More knowledge on the duration of the fertility-enhancing effect of oil-based contrast might contribute to the understanding of the underlying mechanism. It was postulated that an effect on the endometrium or on the immune response in the peritoneum would be short lasting, and that dislodging of mucus or debris from the proximal parts of the Fallopian tubes might be painful but longer lasting (i.e. over multiple cycles). This information can contribute to the search for the mechanism underlying the fertilityenhancing effect of oil-based contrast. The present study investigated whether or not the fertility-enhancing effect of HSG using oil- versus water-based contrast would change over time. MATERIALS AND METHODS The H2Oil study was a multicentre RCT comparing oil-based and water-based contrast in women scheduled for HSG during their fertility workup (Netherlands Trial Register [NTR] 3270) and was approved by the Institutional Review Board of the Amsterdam University Medical Centre – Academic Medical Centre (reference 2008.362, dated 12 February 2009). The H2Oil follow-up study assessed the long-term outcomes of the H2Oil trial (NTR 6577) and was approved by the Institutional Review Board of the Amsterdam University Medical Centre – VU University Medical Centre (reference 2017.221, dated 14 June 2017). Study details and results have previously been published (4, 7). In short, the H2Oil trial recruited a total of 1119 participants in a network of 27 hospitals in the Netherlands between 3 February 2012 and 29 October 2014 (4). Participating infertile women were aged between 18 and 39 years, had an ovulatory cycle, had a low risk of tubal pathology according to their medical history, were without known endocrine disorders and had partners that had a total motile sperm count after sperm wash of more than 3 million/ ml. They had been trying to conceive for at least 1 year and were scheduled for tubal patency testing with HSG at the end of the fertility workup. After informed consent, couples were randomized for HSG with oil-based contrast or water-based contrast. In the H2Oil follow-up study, data regarding fertility treatments and pregnancies were collected until 3–5 years after randomization (7). Study outcomes The main outcome was ongoing pregnancy, defined as an ultrasound-confirmed positive heartbeat beyond 12 weeks of gestation. Additional to various other pregnancy outcomes, data on fertility treatments were collected. The start of follow-up was defined as 2 weeks before HSG (reflecting the first day of the menstruation before HSG). Time to pregnancy was defined as 2 weeks before HSG to the first day of menstruation before
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