I went skiing in Meribel while pregnant 2 years ago, between weeks 11 and 13. For the 2nd week I was with 2 GPs (my best friend and her dh - plus their kids). Lauren's only concern was that I might "pop a hip" (because of the extra relaxin and resultant flexibility). Mind you, it didn't worry her enough to stop me "helping" her beginner daughter to ski (first time on snow, Saturday before ski lessons started), who gave up trying and whom I had to get down the green slope (the Altiport, if you've been to Meribel before) by using MY snow plough to turn HER skis! Having said that, I coped - and that year I had the best, deepest snow plough ever! Great for demonstrating to the kids (once they had some proper lessons) - I was Mother Duck with the ducklings following down Les Creux. (Dh, although a decent skier, has never been able to snow plough!)
My ski instructor was more nervous - he had kittens when I had a minor "sit down" fall in soft snow and stopped doing that particular exercise (much to dh's disgust). However, this year the boot is on the other foot, as his wife (also a ski instructor) is pregnant with twins and is planning on continuing well into the 2nd trimester, although only with beginners. It'll be interesting to see if she is still as confident when we go out at the end of January - but I am sure she will "listen" to her body.
My GP was happy with me going - as he knew I was a confident skier and everything was going well. I skied well within myself - and has a result my skiing probably improved! I din't have a single major fall (I wish the same could be said about last year!) - and still really enjoyed myslef.
However, my experience is "first" trimester. As my GP friends said, everything at that stage is protected behind the pelvis. By 20 weeks, you will definitely be "protruding" AND your centre of gravity will have changed. You need to be careful about that - and be prepared to be flexible (not too literally!).
Although all the American web-sites will say absolutely no (but then they even say to stop doing aerobics, cycling - anything where you might get sued because something might happen which you might then blame on the advice you were given), I have read somewhere (I think it was in the book "The Complete Skier" that it's actually safer in the 2nd than the first trimester. I'm not sure why - maybe to do with having adjusted to the pregnancy?
Anyway, you have to listen to your own body and to the advice of the professionals looking after you. If you feel good, everything is going well, and you're a confident skier, I would give it a go. But you're the only one that can make that call!