Some semen abnormalities may cause infertility by impairing implantation rather than fertilization.

The hypo-osmotic swelling (HOS) test measures the functional integrity of the sperm membrane. Although, the sperm membrane is essential for the fertilization of oocytes, several clinical studies suggest that abnormally low HOS test scores do not predict poor or failed fertilization in human in-vitro fertilization trials. However, in-vivo and in-vitro studies clearly demonstrate that a low HOS score is associated with poor pregnancy rates suggesting this sperm defect causes implantation problems rather than fertilization problems. The problem of implantation could be caused by the supernumerary sperm attached to the zona pellucida. Supporting evidence for this theory was demonstrated by finding high pregnancy and implantation rates despite low HOS scores following intracytoplasmic sperm injection which avoids the presence of supernumerary sperm on the zona pellucida. These data thus support theories that some sperm abnormalities may reduce fertility potential by causing implantation disorders rather than problems with fertilization.