Is it safe to mountain-bike during pregnancy?
Yes, but remember that mountain biking has a certain amount of inherent risk whether you're pregnant or not. And the further along you are in your pregnancy, the greater the risk to your baby should you fall off the bike. A fall could cause trauma to your abdomen, which could lead to premature separation of the placenta, premature labor, or in the worst case, fetal death.
While biking is fine for most people, if you have placenta previa (which means the placenta is covering the cervix), you wouldn't want to do it because the bumping up and down could cause bleeding problems. And as you get further along in your pregnancy it'll be harder to bike and riskier. In the last trimester, it's more difficult to balance and there's more risk that you could fall off the bike. You might want to consider riding a stationary bike as an alternative during pregnancy.