The most common cause of male infertility is decreased number and motility and/or abnormal shape of the spermatozoa in the semen. This is termed oligo-asteno-teratospermia (OATs). The resaon is usually not clear. If there is an accompanying varicocele its treatment is recommended by urologists but the effectiveness of the treatment has not been proven.

Genetic factors may be responsible when the sperm count is < 1 mil/mL. A history if orchitis after mumps, trauma to the testes, previous radio-chemo therapy, late descended testes, and unilateral orchiectomy (removal of the testis) may all cause OATs. If the number of motile sperms per milliliter is over 5 million (number/ml x number of sperms with forward motility) intrauterine insemination (IUI) could be succesful. If no pregnancy occurs after 3-4 IUI treatments intracytoplasmic sperm injection (ICSI) should be done.