This is usually due to an excess in acidity in the mucus (pH<6) although in some cases the mucus secreting cells have been destroyed by surgery, such as treatment for abnormal pap smears or prolapse surgery.   Rarely, mucus cannot be penetrated by sperm because of sperm antibodies in the mucus.

Cervical Mucus Hostility is a relative cause of infertility; that is to say, hostile mucus may reduce the chance of normal conception, but not eliminate it.

Couples with otherwise unexplained infertility but hostile mucus have a 2% chance of normal pregnancy per month.   With artificial insemination by husband (A.I.H.) this chance increases to 10-15% per month, because the mucus is bypassed by placing the sperm directly into the uterus.