In normal fetal development, the testicles develop in the abdomen and descend into the scrotum during the last months before birth. In a small percentage (3.4%) of newborns, one or both testicles fail to descend into the scrotum. In 80% of cases, the undescended testicle migrates into the correct position without intervention during the first year. The condition occurs in both testicles in about 10% of cases.
This condition is known as "cryptorchism". Failure to correct this condition is met with a significant increase in the risk of malignancy (testicular cancer) in the undescended testicle.
The prodecure to correct cryptorchism is called "orchidopexy". An inguinal incision is made and the undescended testicle, with its vascular and vas deferens pedicle are tunneled into the scrotum.
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http://www.nlm.nih.gov/medlineplus/ency/article/003002.htm