Sijstermans et al. 15 compiled a systematic review estimating the prevalence of cryptorchidism by different ages and birth weights (Table 1). They identified 97 articles, but only 49 remained eligible. These studies were conducted between 1934 and 2006. Thirty-eight studies (83%) were prospective, and the other eleven were retrospective, totaling over 704,000 males. Fifty percent of the studies used a formal definition to identify and diagnose cryptorchidism, although these definitions varied widely. Ten percent of articles used the definition by Scorer 16 that considers all testes at least 4 cm below the pubic crest in full term males ( cm in preterm males) as descended; 41% included location in the definition, and 13% excluded high scrotal testes.