Short answer: Don't do it.
Longer answer: Risk factors exist on the slopes that are beyond the control of even the best skier or rider, and the consequences of a fall or collision can be serious on mother and baby alike.
Downhill skiing or snowboarding are among the sports that doctors tend to discourage for pregnant women, even though babies are well-protected when in the womb.
Skiing and snowboarding involve speed, momentum, slippery surfaces, other skiers and riders, and hard objects like trees, lift towers, and the trail surface itself.
Specific dangers from falls include placental abruption or induction of preterm contractions, both of which can lead to premature delivery.
Even without a collision with immovable objects or other skiers or riders, risks exist for pregnant women and their babies.
Balance tends to be off because the center of gravity shifts as pregnancy progresses, leading to increased possibility of falls which, again, are not good.
There's also measurably less oxygen at the altitudes found in higher mountains, with possible impact on the amount of oxygen the baby receives.
There is, moreover, an entire lifetime of skiing or riding ahead and the loss of one season is probably small price to pay for peace of mind, and for the health and well-being of yourself and your baby.
Can't stand the thought of a winter without getting on the snow? Try snowshoeing. It's really fun.
Question: Is it safe to ski while pregnant?
Related Question: Skiing when pregnant.?