I must admit that I haven't much idea of how the thing works, when you're a girl they tell you it's the eggs that don't get fecondated that have to just get out some way, and here is the cycle. Could be all wrong, as most of things they tell you.
But I still know that in the taoist communities, where tantric practices were a very essential part of the practices of inner alchemy, to cut the red dragon was also meant to not get pregnant, besides the non-ejaculation which was what practiced by men (and which is not safe at all, as there is still possibility of some leaking). In fact there were few babies around, but not as many as they could have been.