Your spermatic cord carries blood to and from your testicles. It's not certain what causes varicoceles, but many experts believe a varicocele forms when the valves inside the veins in the cord prevent your blood from flowing properly. The resulting backup causes the veins to widen (dilate).

Varicoceles often form during puberty. Varicoceles usually occur on the left side, most likely because of the position of the left testicular vein. However, a varicocele in one testicle can affect sperm production in both testicles.

Jan. 10, 2012