I didn't mountain bike when I was pregnant 13 years ago, but I commuted by bike and rock climbed. My doc was pretty old school, but still she told me to continue doing the activities I was competent in, not take up new ones or compete, avoid obvious high risk or strenuous effort, stay hydrated and and otherwise relax
Check this with your doc, but I'm pretty sure that the risk of miscarriage from bumping around is pretty much a myth, though an understandable one. Anything putting an obvious strain on you can be a risk though, like very high effort, overheating and crashing, of course.
At the end of my pregnancy I found biking more comfortable than walking, because it would ease the pressure on my perineum. This was just easy city riding, though.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett