The cryptorchidism prevalence among infants in the general population of Rotterdam, the Netherlands.

Published trends and geographical differences in cryptorchidism rates are almost exclusively derived from hospital-based birth defect registers, which are sensitive to selection bias and incomplete reporting. This study aimed to accurately assess the cryptorchidism prevalence in the general population of Rotterdam. Of 7652 consecutive male live births, 7292 (95%) were examined for cryptorchidism at Child Healthcare Centres around the age of 1 month. In a subgroup of cases, the persistence of cryptorchidism was re-assessed during a follow-up examination by expert specialists. The cryptorchidism rate at the median age of 35 days was 1.2% (89/7292). In the re-examined subgroup (median age 95 days) 69% of the boys (24/35) had persistent cryptorchidism, of which 20 were unilateral and four bilateral. The population rate of 1.2% falls within the range of 0.9-9% reported by others. Differences in case ascertainment and population characteristics probably explain part of the differences between studies. Our cross-sectional design does not allow for analysis of a temporal trend, but provides a baseline for future trend studies. To study cryptorchidism rates, trends, and risk factors, a systematic case ascertainment is warranted.