The testicles are the male sex organs that produce sperm and the male hormones. They form near the kidneys. As the fetus grows in the mother's womb, the testicles move down through the groin into the scrotum. Normally they are in the scrotum before birth. Sometimes a testicle doesn't fully descend into the scrotum before birth. Instead, it stops somewhere along the normal pathway between the kidney and the scrotum. Or it may stray from this pathway. What causes this isn't known. An undescended testicle is most common in premature babies and most often only one testicle is affected. Treatment is important, because the testicle doesn't descend on its own, it should be treated. The longer a testicle remains outside the scrotum, the more likely it is that it will produce fewer sperm. An undescended testicle has a higher risk of cancer. This is true even after the testicle is brought down into the scrotum. Bringing the testicle down makes a problem easier to find. An undescended testicle can leave a small tear or hernia in the wall between the abdomen and the groin. The hernia needs to be treated to prevent future problems.