According to this abstract, the IOC originally banned norethindrone NOT because it is performance enhancing in itself, but because some drug testing labs couldn't distinguish it from nandrolone, an anabolic steroid.

Interestingly, the Snopes entry on abortion doping suggests that it's the abortion, not the initial pregnancy, that is supposed to be performance enhancing. (Now, I consider Snopes only marginally more reliable than the rumors it's supposed to debunk, but FWIW, she lists that one as "indeterminate.")

I've never taken it myself, so can't add anything personal.