Not seeing or feeling a testicle where you would expect it to be in the scrotum is the main sign of an undescended testicle.

Testicles form in the abdomen during fetal development. During the last couple of months of normal fetal development, the testicles gradually descend from the abdomen through a tube-like passageway in the groin (inguinal canal) into the scrotum. With an undescended testicle, that process stops or is delayed.

The exact cause of an undescended testicle isn't known. A combination of genetics, maternal health and other environmental factors might disrupt the hormones, physical changes and nerve activity that influence the development of the testicles.

Factors that might increase the risk of an undescended testicle in a newborn include:

• Low birth weight
• Premature birth
• Family history of undescended testicles or other problems of genital development
• Conditions of the fetus that can restrict growth, such as Down syndrome or an abdominal wall defect
• Alcohol use by the mother during pregnancy
• Cigarette smoking by the mother or exposure to secondhand smoke
• Parents' exposure to some pesticides

In order for testicles to develop and function normally, they need to be slightly cooler than normal body temperature. The scrotum provides this cooler environment. Complications of a testicle not being located where it is supposed to be include:

• Testicular cancer. Testicular cancer usually begins in the cells in the testicle that produce immature sperm. What causes these cells to develop into cancer is unknown. Men who've had an undescended testicle have an increased risk of testicular cancer.

The risk is greater for undescended testicles located in the abdomen than in the groin, and when both testicles are affected. Surgically correcting an undescended testicle might decrease, but not eliminate, the risk of future testicular cancer.

• Fertility problems. Low sperm counts, poor sperm quality and decreased fertility are more likely to occur among men who've had an undescended testicle. This can be due to abnormal development of the testicle, and might get worse if the condition goes untreated for an extended period of time.

Other complications related to the abnormal location of the undescended testicle include:

• Testicular torsion. Testicular torsion is the twisting of the spermatic cord, which contains blood vessels, nerves and the tube that carries semen from the testicle to the penis. This painful condition cuts off blood to the testicle.

If not treated promptly, this might result in the loss of the testicle. Testicular torsion occurs 10 times more often in undescended testicles than in normal testicles.

• Trauma. If a testicle is located in the groin, it might be damaged from pressure against the pubic bone.
• Inguinal hernia. If the opening between the abdomen and the inguinal canal is too loose, a portion of the intestines can push into the groin.