Objective Adverse maternal lifestyle and environmental exposures during pregnancy are suspected to interfere with the normal testicular descent and increase the risk of cryptorchidism in the offspring. We studied concordance rates (i.e. the probability that a pair of individuals will both have cryptorchidism, given that one of the individuals has the condition) in pairs of boys with varying genetic relation, to evaluate the risk contribution from the environment in utero and the genes respectively.

Methods A population of 1,079,780 Danish boys born from January 1, 1973 to December 31, 2004 was used. Six groups of paired boys were identified: Randomly selected non-related boys (different parents), paternal half brothers (same father, different mothers), maternal half brothers (same mother, different fathers), full brothers (same parents), dizygotic twin brothers and monozygotic twin brothers. Status on each individual regarding diagnosed cryptorchidism and orchiopexy procedures were obtained from the Danish National Patient Register.

Results Concordance rates of diagnosed cryptorchidism in the groups were as follows: Randomly selected non-related boys 3.2% (95% CI 2.7 to 3.6), paternal half brothers 3.4% (2.3 to 4.7), maternal half brothers 6.0% (4.5 to 7.7), full brothers 8.8% (8.3 to 9.8), dizygotic twin brothers 24.1% (16.0 to 33.6) and monozygotic twin brothers 27.3% (15.5 to 41.2). A similar pattern was observed when orchiopexy procedures were studied.

Conclusions The concordance rate was higher among maternal half brothers than paternal half brothers, and of equal magnitude in both twin groups. The findings strongly support that the environment in utero is a significant contributor to the appearance of cryptorchidism.