It reflects the physiologic situation in vivo and assess both the quality of cervical mucus and the penetration ability of sperm. The quality of cervical mucus varies during the cycle and is favourably influenced by estrogens, becoming more abundant and fluid at the time of ovulation. Therefore post-coital tests are scheduled just before ovulation as determined by basal body temperature, or more accuratly by follicular sizing by ultrasonography. The number of motile sperm per high power microscopic field is recorded and the test is considered positive when 10 or more motile sperm are found per field according to WHO guidelines. A decreased number of sperm in cervical mucus when the cervical mucus score is low reflects inadequate mucus rather than impaired sperm function.
Cervical mucus is colonized by sperm that are stored for several hours in cervical crypts.
Sperm cells then gradually migrate trough the cervix. The post-coital test has been used for several decades but is reported to be inaccurate and to lack consensus normal values and methodology. Part of the problem may be due to the non-homogeneous nature of the cervical mucus that prevents quantitative determination of sperm concentration. The post-coital test however remains an inexpensive and noninvasive procedure that gives information about the occurrence of ejaculation and the ability of the sperm cells to function within the cervical environment. This test can be performed in parallel with donor semen or donor cervical mucus, and therefore allows to discriminate between a male versus female origin of the sperm immobilizing factor.