OBJECTIVE To investigate the fetal development of the internal urethral sphincter and the gender-related morphologic differences of the bladder outlet. METHODS Thirty-seven (14 female, 23 male) fetal bladder neck specimens (mean gestational age, 19.4 weeks) with the smooth muscle complex of the internal sphincter were investigated histologically. After immunostaining serial sections in 3 reference planes (sagittal, frontal, and horizontal) of the bladder neck, the internal sphincter volumes and bladder outlet diameters were measured and correlated with gender and age of gestation. RESULTS Between the 18th and 40th week of gestation, an exponential growth of the internal sphincter muscle with significant higher volumes could be observed in male fetuses compared with female fetuses (internal sphincter volumes, P = .006; radius of the sphincter complex, P = .001). As a result of this gender difference, the bladder outlet was significantly (P = .001) narrower in male than in female fetuses. Moreover, we found a significant positive correlation between age and all measured parameters in both male and female specimens. CONCLUSIONS The present study indicates a significant closer bladder outlet in male fetuses compared than in females. It thereby provides evidence of a gender-related functional obstruction in addition to a suppositious transient infravesical obstruction in male human fetuses.