If you're afraid you'll spontaneously combust if she's got a bun in the oven, don't go. Won't be able to tell in the early weeks/months, and it's one of the many personal things you'd be silly to think she'll tell you.
I prefer dancers who clearly have puberty in the rear view. That rarely goes together with a gym rat flat stomach. So the "not visibly pregnant" phase lasts longer in my eyes.
I had a couple favs with whom I agreed to keep going "till they showed". But (of course), they put me in the position of having to be the one to say, yeah, no longer looks like you've just been hitting Olive Garden a little too hard.
I did try one dancer's breast milk. Not from her boobs, she put it in a cup. She wanted me to tell her how it was, because she was too grossed out to try it herself.
Back in my escort days, one of my favs told be she was no longer doing BBBJ, because she was expecting. Of course, didn't tell me when we made the appointment.