When a couple conceives naturally, semen is deposited in the vagina and the sperm enters the female reproductive tract by swimming through the cervical mucus contained in the cervical canal. However, this process can fail to work for a number of reasons and require clinical intervention.
These difficulties can include an absence of semen (the man is away at the time of ovulation, or the woman may not have a partner), the semen may contain no sperm (the man produces azoospermic semen), or the cervical mucus may be hostile to the sperm.
These problems can sometimes be overcome by the use of washed sperm (from either the partner or a donor) being deposited into the uterus using a catheter, thereby by-passing the cervical canal. The sperm are washed using density gradients and centrifugation in the andrology laboratory.