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Study Question: Does intrauterine insemination in the natural cycle lead to better pregnancy rates than intracervical insemination (ICI) in the natural cycle in women undergoing artificial insemination with cryopreserved donor sperm., Summary Answer: In a large cohort of women undergoing artificial insemination with cryopreserved donor sperm, there was no substantial beneficial effect of IUI in the natural cycle over ICI in the natural cycle., What Is Known Already: At present, there are no studies comparing IUI in the natural cycle versus ICI in the natural cycle in women undergoing artificial insemination with cryopreserved donor sperm., Study Design, Size, Duration: We performed a retrospective cohort study among all eight sperm banks in the Netherlands. We included all women who underwent artificial insemination with cryopreserved donor sperm in the natural cycle between January 2009 and December 2010. We compared time to ongoing pregnancy in the first six cycles of IUI and ICI, after which controlled ovarian stimulation was commenced. Ongoing pregnancy rates (OPRs) over time were compared using life tables. A Cox proportional hazard model was used to compare the chances of reaching an ongoing pregnancy after IUI or ICI adjusted for female age and indication., Participants/materials, Setting, Methods: We included 1843 women; 1163 women underwent 4269 cycles of IUI and 680 women underwent 2345 cycles of ICI with cryopreserved donor sperm., Main Results and the Role of Chance: Baseline characteristics were equally distributed (mean age 34.0 years for the IUI group versus 33.8 years for the ICI group), while in the IUI group, there were more lesbian women than in the ICI group (40.6% for IUI compared with 31.8% for ICI). Cumulative OPRs up to six treatment cycles were 40.5% for IUI and 37.9% for ICI. This corresponds with a hazard rate ratio of 1.02 [95% confidence interval (CI) 0.84-1.23] after controlling for female age and indication. Increasing female age was associated with a lower OPR, in both the IUI and ICI groups with a hazard ratio for ongoing pregnancy of 0.94 per year (95% CI 0.93-0.97)., Limitations, Reasons for Caution: This study is prone to selection bias due to its retrospective nature. As potential confounders such as parity and duration of subfertility were not registered, the effect of these potential confounders could not be evaluated., Wider Implications of the Findings: In women inseminated with cryopreserved donor sperm in the natural cycle, we found no substantial benefit of IUI over ICI. A randomized controlled trial with economic analysis alongside, it is needed to allow a more definitive conclusion on the cost-effectiveness of insemination with cryopreserved donor sperm., Study Funding/competing Interests: No funding was used and no conflicts of interest are declared., (© The Author 2015. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)