Sperm are mixed into semen at the time of ejaculation. Natural enzymes contained in semen take about a half hour to break it down from a relatively firm, sticky coagulum to a more watery state. Sperm can then migrate out of it into the cervix where they begin their journey through the woman’s reproductive tract.

In the case of IUI, liquefied semen is layered on top of a sterile solution at the bottom of a centrifuge tube and spun at high speed for several minutes. The solution is denser than water, so only motile sperm can migrate into it – the centrifugation merely accelerates the process. In this density gradient solution, the motile sperm are in effect, weightless, minimizing damage done by centrifugation. The small fraction of ejaculated sperm at the bottom of the density gradient is highly enriched for vigorous, motile sperm.

This fraction is mixed into a small volume of nutrient tissue culture media, and is directly injected into the patient’s uterine cavity via the cervix.