We know from the scientific research of Professor Erik Odeblad that cervical mucus is essential for fertility.  One type of mucus protects and nourishes the sperm so that they retain their fertilising capacity.  Another mucus acts as a filter, destroying damaged sperm cells. Yet another forms channels which help the sperm travel through the woman’s reproductive system to meet and fertilize the ovum.  Even mucus outside the vagina can enable sperm cells to reach the egg.  (Thus pregnancy can result from genital contact without full penetration – withdrawal of the penis before ejaculation is not a reliable method of avoiding pregnancy.)

During the times of infertility the cervix is blocked by a thick plug of different mucus which is impenetrable to sperm causing them to die quickly.