I’m pretty sure that we all know what spell Trixie’s referring to when she mentioned how she got pregnant…the good old gender-bend spell where for a fixed set of time, any pony can turn from mare to stallion-and back, do the deed when the partner is in heat and Bob’s your uncle–pregnant mare. And the kicker–it doesn’t matter which mare can be gender-bended…so that means that Twilight and Trixie can take turns being the ‘stallion’. And unlike other magic spells, this particular genderbend spell is fully fertile…
 
So, congrats Trixie!! And congrats to AJ and Rainbow when that happens. (Look I’m already seen the entire story, so no complaints of spoilers–unless you’re never seen this before, which if that is the case, sorry)