The formation of the male urethra in utero is complex, and not completely understood. This development can go awry in a number of ways, including part or all of the urethra in the wrong location, valves blocking urine flow, or dilatation so that flow isn't blocked when it should be. Some boys even have two urethras. Urethral duplications occur in many varieties and seem to come from many different abnormalities of development. No widely accepted classification has yet been adopted.
Most duplications occur with one urethra on top of the other.
Occasionally, however, duplex urethras occur side by side. This is usually the case if a child has two complete penises, but also occurs if the penis is fused but widened. Most of these children have two bladders, plus two normal urethras, and only need surgery if it is needed for cosmetic reasons. The more common situation of one urethra on top of another occurs in dorsal and ventral varieties. In dorsal duplications of the urethra, a normal urethra follows the usual channel and ends in a normal hole at the tip of the penis.