Which exercise is recommended depends on the person's fitness level. But in general, women should continue the exercise they already do – it will just become harder as they carry extra weight. And for those who don't do any exercise? Walk. "Put on a pedometer, that'll get you going," Riley says.
As for moves to steer clear of, avoid exercising from a flat-on-the-back position, which can cause back strain and impede blood flow that could lead to fainting, Harms says. Also, there's no point in doing abdominal work, since these muscles are being stretched to accommodate the baby, he says. Plus, you may want to reconsider workouts that hinge on balance, since a redistribution of weight can increase the odds of injury. "You do get clumsy," he says. "If you have always been a biker, consider a stationery bike."
You'll also want to avoid contact sports for obvious reasons. "Any sport where you have a high likelihood of falling and hitting your abdomen, that's going to be bad," Riley says. "Other than that, you can do almost anything, and people who are sort of in tune with their bodies will modify their exercise as the pregnancy progresses."
[Read: Prenatal Yoga: What You Should Know.]