Why birth control "doesn't work"

It is pretty common knowledge that birth control doesn't work on X-Change, but the why is a little more interesting. On Basic you don't need birth control because the body created doesn't have the natural hormones to ovulate present so pregnancy can't occur. Knockoffs sometimes use things to "kickstart" the sex drive that also causes ovulation and thus the increased pregnancy risk. Xtra gives your body long enough to start producing those hormones so there is a non-zero chance that you will ovulate, though it is lower for a natural woman (for the first month) and the same as an 18-year-old woman in perfect health after that (roughly 90% chance)

All of that is very interesting but the mechanism that turns you back into a man checks if you have pregnancy hormones, like the ones in birth control, and if you do it makes the change permanent. That being said you wake up a woman, the pregnancy tests all show negative but you have managed to prevent yourself from changing back.