69 IUI success and prognosis of natural conception 4 prognosis and landmark time point to the previous model to see if the effect modification of prognosis on IUI-OS changed over mimicked trials i.e. when starting treatment later. We used Akaike’s Information Criterion (AIC) (at least two points difference) and Wald tests for the interaction terms to determine whether including the interactions resulted in a better fit of the model to the data (23). Missing data Data were missing on duration of subfertility (n=3), referral status (n=2), primary or secondary subfertility (n=1), the percentage of progressive motile sperm (n=81) and total motile sperm count (n=93) and was accounted for using single imputation. All statistical analyses were performed using R version 3.3.2 24)(R Core Team (2017). http://www.Rproject.org/.) using the survival, dynpred, mice and CreateTableOne packages. RESULTS Out of 1119 couples included in the H2Oil trial, we selected 975 for analysis after excluding couples with other diagnoses than unexplained subfertility, couples who conceived before HSG and couples with missing outcome data. Of these 975, 587 couples (60%) received 2386 IUI cycles after HSG, of whom 221 couples conceived after IUI, leading to ongoing pregnancy (rate: 0.74 per couple per year over a median follow up for IUI of 5 months). Out of 388 couples (40%) followed up for one and a half years of expectant management after HSG, 299 conceived naturally leading to ongoing pregnancy (rate: 0.56 per couple per year over a median follow-up of 4 months). In total, 62 couples out of 587 (11%) who underwent IUI-OS started treatment directly after HSG and the remaining 525 (89%) first had a period of expectant management. The median period between HSG and starting IUI-OS was 4 months. A total of 1723 (72%) IUI cycles used OS. Forty-two couples (4%) received IVF as their first treatment, with a median period of expectant management of nine months between completion of the fertility workup and the start of IVF. We depicted the number of couples followed under expectant management or followed under IUI-OS over time in Figure 1. Until approximately half a year of follow up the number of couples who were currently in an IUI-OS treatment pathway kept increasing, after which this number declined again.
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