29 Duration of the fertility enhancing effect of HSG with oil-based contrast 2 Figure 1. Kaplan-Meier curve for time to an ongoing pregnancy. Primary analysis The hazard ratio for ongoing pregnancy estimated over 3 years follow-up of oil-based versus water-based contrast was 1.26 (95% CI 1.10–1.45). Using regular time, the scaled Schoenfeld residuals plot showed a slight decrease in hazard ratio over time from approximately 1.6 after HSG to 1.3 after 1 year (Supplementary Figure 1, test for nonproportional hazards over time: P = 0.10). Using log-transformed time, the decrease in log hazard ratio over time was more pronounced shortly after HSG and was statistically significant (Supplementary Figure 2, test for non-proportional hazards over time: P = 0.02). There thus seems evidence that the effect of oil versus water decreases over time, and that this occurs mostly within the first year after HSG. The best fitting model included a linear term for the interaction between log-transformed time and allocation (P = 0.02 for interaction, an approximately 3-point decrease in AIC compared with the model with only a main treatment effect). The estimated hazard ratios at different follow-up time points are shown in Table 1. The estimated hazard ratio of oil versus water started at 1.71, gradually decreased over follow-up time and eventually plateaued around 1 after approximately 2 years of follow-up.
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