66 Chapter 4 of last menstruation before starting IUI or IVF or, in case they conceived naturally, the first day of the last menstruation before conceiving. The endpoint was ongoing pregnancy, defined as the presence of foetal cardiac activity at transvaginal sonography at a gestational age of at least 12 weeks (11). Couples who miscarried before 12 weeks were not censored since they could still achieve ongoing pregnancy in subsequent cycles after their miscarriage. If no ongoing pregnancy occurred, we censored follow up at the end of expectant management or, if treated, at the end of the IUI period. Cumulative pregnancy rates over multiple IUI cycles We used the same statistical approach as in our previous study (7). In short, we used the sequential Cox approach to compare multiple cycles of treated and untreated couples, not only directly after completion of the fertility workup but also if they started later (13). In this approach, we derived multiple datasets from the cohort in which couples started IUI-OS at approximately the same point in time and compared them to couples undergoing expectant management at that time, ‘mimicking’ hypothetical RCTs (13). At completion of the fertility workup and each consecutive month thereafter, named the landmark time points, we constructed such a mimicked trial from our data in which we included all couples who remained in the cohort i.e. couples who had not conceived, had not started treatment and were not lost to follow up before that landmark time point. In these ‘trial’ datasets spanning 6 months, we considered couples as treated if they started IUI-OS early i.e. within one month after the landmark time point. Couples who did not start IUI-OS within the first month were used as controls. Couples who started IUI-OS within the 6 month window of a trial, but later than one month after the landmark time point, were counted as controls during their untreated period and ‘artificially censored’ at the time of starting IUI-OS. This way, couples were not included in a single group throughout the study. Instead, couples who at some point started IUI-OS were analysed as controls (under expectant management) in the ‘mimicked’ trial datasets preceding the month in which they started IUI-OS. When they started IUI-OS, their following treatment cycles were analysed as part of the treated (IUI-OS) group in the mimicked trial dataset that started that month. In order to compare results to the previous study, we restricted our data to a maximum of eighteen months of follow up.
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