The condition is thought to be caused by under-stimulation of the genital tubercle and penis by dihydrotestosterone, resulting in failed fusion of the urogenital folds. However, deficiency or abnormality of testosterone, dihydrotestosterone or 5-α reductase is not commonly found in hypospadias alone. Direct aetiology is unknown in most cases and it appears to be a multifactorial process with links made to hormonal changes, genetic sus-ceptibility and environmental factors.Female sex hormones have been implicated and the ever increasing use of oestrogens has been suggested as a possible cause for the recent rise in incidence. In addition it is five times more common in boys born from IVF – possibly due to the effects of materal progesterone on 5-alpha reductase.

Monozygotic twins have an 8 times increased risk of hypospadias compared to single-tons. This may be due to in-utero competition for HCG. In some cases there also appears to be a genetic link. Of affected boys, 8% have affected fathers and 14% have affected brothers. If two family members are affected, the risk for a subsequent boy is 22%.