Live birth after intrauterine insemination: is there an upper cut-off for the number of motile spermatozoa inseminated?

To date, most studies have investigated the minimum number of spermatozoa available for intrauterine insemination (IUI), with no data on the maximum number of motile spermatozoa inseminated (NMSI) having been published. This study aimed to determine whether an upper cut-off for the NMSI during IUI exists above which the live birth rate (LBR) is negatively affected. Retrospective analysis of autologous IUI cycles performed between January 2010 and July 2018 in women <43 years old with a NMSI >1 million. The main outcome was the LBR per IUI cycle as a function of the NMSI. A total of 2592 IUI cycles performed in 1017 couples were included. The LBR increased with NMSI up to 30 million without any upper threshold (AUC = 0.5441). The LBR per IUI cycle were 14.5%, 17.9% and 22.7% for NMSI of >1 to ≤10, >10 to ≤20 and >20 to ≤30 million, respectively (P = 0.003). By univariate analysis, the NMSI, female age, number of mature follicles and oestradiol concentrations on day of ovulation triggering, cycle number and infertility aetiology influenced the LBR. Multivariate analysis showed that the LBR was 1.49 and 1.78 times higher when IUI was performed with a NMSI >10 to ≤20 million (odds ratio [OR] 1.49; 95% confidence interval [CI] 1.10-2.02]) and >20 to ≤30 million (OR 1.78; 95% CI 1.08-2.94), respectively, compared with IUI with a NMSI >1 to ≤10 million. The LBR after IUI can be optimized by inseminating a maximum of motile spermatozoa up to 30 million. Thus, in this specific cohort, IUI preparations should not be diluted when more than 10 million motile spermatozoa are obtained.

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