 The main cause of infertility in men with CF is congenital absence of the vas deferens (which normally connects the testes to the ejaculatory ducts of the penis), but potentially also by other mechanisms such as causing no sperm, abnormally shaped sperm, and few sperm with poor motility.Many men found to have congenital absence of the vas deferens during evaluation for infertility have a mild, previously undiagnosed form of CF.Around 20% of women with CF have fertility difficulties due to thickened cervical mucus or malnutrition.