65 IUI success and prognosis of natural conception 4 RCT on contrast fluid used for hysterosalpingography (HSG). This is a different approach to see if the effectiveness of IUI-OS here also depends on the prognosis of natural conception, as well as to validate the thresholds of 25% and 40%. MATERIALS AND METHODS The H2Oil study was a multicentre RCT to compare ongoing pregnancy rates in subfertile women who underwent HSG with oil-based versus water-based contrast (Netherlands Trial Register number, NTR3270). The study was approved by the Institutional Review Board of the Amsterdam University Medical Centre – Academic Medical Centre (reference 2008.362, dated 12 February 2009). The study details and results have been published previously (11). The H2Oil follow-up study (NTR 6577) assessed the long term outcomes until 3–5 years after the H2Oil study and was approved by the Institutional Review Board of the Amsterdam University Medical Centre, location VU University Medical Centre (reference 2017.221, dated 14 June 2017). Study details and results have been published elsewhere (12). Inclusion and exclusion criteria Included women were between 18 and 39 years of age, were having ovulatory cycles and had a low perceived risk of tubal pathology based on their medical history. They had tried to conceive unsuccessfully for at least 1 year and had an indication for tubal patency testing. Exclusion criteria were known endocrine disorders and having a partner with severe male subfertility (defined as a total motile sperm count after sperm wash of less than 3 million per millilitre). Women were randomized for an HSG with oil-based contrast or an HSG with water-based contrast. Data regarding fertility treatments and pregnancies were collected until 3–5 years after randomization. Follow up and outcome definitions For the follow up of selected couples, we distinguished between time spent pursuing expectant management and time spent receiving IUI-OS cycles. The start of the IUI period was defined as the first day of menstruation before the first IUI cycle. The end of the IUI period was defined as the first day of menstruation before the last IUI cycle. All pregnancies in the IUI period thus resulted from IUI. Follow up for expectant management started 14 days before they received HSG and ended at the last date of contact, first day
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