"It's true you don't see many Uruk-hai women. But in fact, they're so alike in voice and appearance, that they're often mistaken for Uruk-hai men. This, in turn, has led to the belief that there are no Uruk-hai women, and that Uruk-hai just spring out of holes in the ground."
This is where the Uruk breeding pits from The Fellowship of the Ring came from.