A varicocele is an enlarged set of veins in the scrotum, most often found on the left side. A varicocele is similar to varicose veins you may see in a leg. Varicoceles can cause low sperm production, low sperm quality, and infertility. Much like in varicose veins, the valves in these veins become damaged and blood begins to pool, increasing the pressure and dilating the veins.