Congenital (present from birth) absence of the two vasa deferentia, which conduct sperm from the testes to the ejaculate; hence a cause of obstructive azoospermia. Because the vas deferens is usually affected on both sides, the word "bilateral" is often inserted, as CBAVD. Infertility is inevitable, but can be overcome using in vitro fertilisation with testicular sperm extraction. The cause, however, is usually the presence among the man's genes of one of the serious alleles for the genetic disease cystic fibrosis (which, having recessive inheritance, results when there are two such alleles present); it can also come about when there are one or two of the less serious abnormal alleles for this condition. The more common of the abnormal alleles should therefore be screened for (using a specially set up polymerase chain reaction on white blood cells): should an abnormal allele be present (delta F508 is the most common seriously abnormal one), the woman should be screened too, to predict the chance of cystic fibrosis occurring in the offspring.

A serious genetic disease (with a recessive inheritance pattern) characterised by a major disturbance of the body's mucus secretions, and thus a cause of incapacitating disease of the lungs. Important in our context because the heterozygous (or carrier) state in men can manifest with azoospermia due to congenital absence of the vasa deferentia -- the infertility of which is nowadays able to be overcome with testicular sperm extraction and ICSI, thus risking inadvertent transmission.

The long duct that transports sperm cells (spermatozoa) from the epididymis to the seminal vesicles. Can be missing from birth (congenital absence of the vasa deferentia) or blocked as a result of infection or intentional interruption (vasectomy). Plural: vasa deferentia.