Sperm are formed in the testes and make their way from there to the epididymis, where they are nourished by fluids produced by the prostate and the seminal vesicles above, the vas deferens, and from mucous glands in the head of the penis.

Factors affecting sperm quality include sperm shape, motility (movement), whether sperm membranes are intact, and the ability of the sperm to bind to and penetrate the egg. Although men produce new sperm throughout their lives, there is also some evidence that sperm quality may decline as men age.