Of course, the prevailing medical wisdom is that exercise is good for pregnant women, particularly those that already exercise regularly before becoming pregnant. Clearly Marla falls in this category. Still, my feeble male mind can't help but think that this has got to be dangerous at this stage in the game.
Rather than allow my neanderthal thinking to overcome me, I looked it up:
My conclusion after reading the articles; it is perfectly valid to continue cycling while pregnant as long as you were an active cyclist before becoming pregnant. Assuming an otherwise healthy pregnancy the most critical health risks appear to be from falling, primarily in the third trimester. An average rider should probably consider riding in more controlled circumstances (trainer, paved bike paths, etc.) to reduce risk. Certainly technical singletrack could be risky.
Of course Marla is not your average rider. Although she appears to be riding on dirt trails, they do not seem to be of the extreme variety she has been known to ride. She's a smart cookie so I am sure Marla is aware of the risks and taking what she considers to be reasonable precautions.
This is one tenacious woman...I'm sure she'll snap back to this kind of shape in a matter of weeks!