A woman’s cervix, the entrance into her uterus, produces mucus which can sometimes be detrimental to sperm and prevent the sperm cells from swimming through the cervix. This problem, sometimes called dysmucorrhea or cervical factor infertility, can result in the cervix acting as a barrier to the sperm. IUI bypasses this problem because the sperm are placed beyond the cervix. - https://fertilitynjny.com/fertility-treatments/intrauterine-insemination/

https://www.merriam-webster.com/dictionary/mucorrhea#:~:text=%3A%20discharge%20of%20mucus%20especially%20when%20excessive