Observations on the epidemiology of simple hypospadias.

A descriptive epidemiological study of simple hypospadias-that is, hypospadias without coexisting malformation outside the genital system-in a birth population of 46,927 singleton males and 1,013 twin males showed that the condition was significantly more common (P < 0.001) in uniovular twin males than in either singleton or binovular twin males. Also the temporal distribution of dates of conception had a significant cyclic pattern for each year of the study. Both observations are thought to support the view that gonadotrophin levels at stage 3 sexual organogenesis play an important part in normal masculinization of male external genitalia.