How is it that on the mini pill cervical mucus becomes too thick for sperm to pass through, but is still thin enough to let menstrual fluid out?

Sperm are delicate!: Thickened of cervical mucus causing a barrier is one of the ways the mini-pill works, but it also inhibits ovulation in about 50% of menstrual cycles. The short answer is that the amount of cervical mucus thickening that results seems to prevent the passage of most sperm, but the mass of menstrual blood flow is stronger & is able to pass. I believe it is a simple force of flow & gravity issue:)