Cryptorchidism is the absence of one or both testes from the scrotum. The word is from Greek κρυπτός (kryptos) hidden and ὄρχις (orchis) testicle. It is the most common birth defect of the male genital tract. About 3% of full-term and 30% of premature infant boys are born with at least one undescended testis. However, about 80% of cryptorchid testes descend by the first year of life (the majority within three months), making the true incidence of cryptorchidism around 1% overall. Cryptorchidism may develop after infancy, sometimes as late as young adulthood, but that is exceptional. Cryptorchidism is distinct from monorchism, the condition of having only one testicle. Though the condition may occur on one or both sides, it more commonly affects the right testis.A testis absent from the normal scrotal position may be:

Anywhere along the "path of descent" from high in the posterior (retroperitoneal) abdomen, just below the kidney, to the inguinal ring
In the inguinal canal
Ectopic, having "wandered" from the path of descent, usually outside the inguinal canal and sometimes even under the skin of the thigh, the perineum, the opposite scrotum, or the femoral canal
Undeveloped (hypoplastic) or severely abnormal (dysgenetic)
Missing (also see anorchia).About two-thirds of cases without other abnormalities are unilateral; most of the other third involve both testes. In 90% of cases, an undescended testis can be felt in the inguinal canal. In a small minority of cases, missing testes may be found in the abdomen or appear to be nonexistent (truly "hidden").
In rodents, androgens induce the genitofemoral nerve to release calcitonin gene-related peptide, which produces rhythmic contractions of the gubernaculum, a ligament which connects the testis to the scrotum, but a similar mechanism has not been demonstrated in humans. Maldevelopment of the gubernaculum or deficiency or insensitivity to either AMH or androgen can, therefore, prevent the testes from descending into the scrotum. Some evidence suggests an additional paracrine hormone, referred to as descendin, may be secreted by the testes. In many infants with inguinal testes, further descent of the testes into the scrotum occurs in the first six months of life. This is attributed to the postnatal surge of gonadotropins and testosterone that normally occurs between the first and fourth months of life. Spermatogenesis continues after birth. In the third to fifth months of life, some of the fetal spermatogonia residing along the basement membrane become type A spermatogonia. More gradually, other fetal spermatogonia become type B spermatogonia and primary spermatocytes by the fifth year after birth. Spermatogenesis arrests at this stage until puberty. Most normal-appearing undescended testes are also normal by microscopic examination, but reduced spermatogonia can be found. The tissue in undescended testes becomes more markedly abnormal ("degenerates") in microscopic appearance between two and four years after birth. Some evidence indicates early orchiopexy reduces this degeneration. Pathophysiology
At least one contributing mechanism for reduced spermatogenesis in cryptorchid testes is temperature. The temperature of testes in the scrotum is at least a few degrees cooler than in the abdomen.