Unlike women, men continue to make reproductive cells or gametes throughout life.  Sperm production takes place in the testicle and is estimated to take about 50 - 70 days. When the production of sperm is complete the sperm are transported to the epididymis where they will remain stored until the next ejaculation. Prolonged storage reduces the quality of the sperm, so more frequent ejaculations can increase male fertility. Once ejaculated sperm can typically survive for 2 days, although under ideal circumstances they may stay alive for up to 5 days.

Several lifestyle factors can affect sperm quality. Alcohol, obesity, excessive caffeine use and any smoking all affect the health of the sperm. In addition, with age there is an increased chance of genetic abnormalities in the sperm. Older men also have lower sperm counts, but sperm production never stops completely.

A semen analysis is best performed after 2 - 3 days of abstinence. The lab scientist will look at several characteristics, but the main ones are the sperm count, the forward movement of the sperm and their shape. It is also important to make sure that the sample is checked for the presence of sperm antibodies. A normal semen analysis is reassuring but does not rule out a male infertility problem. Sometimes sperm cannot fertilise the egg and this is something that can't be detected on a semen analysis.