That could mean walking or yoga - but probably not skiing or horseback riding. Langer adds, "If you are a trained athlete - used to vigorous activity - then you can have your heart rate higher than someone who is very sedentary and didn't exercise at all before pregnancy. The recommendation by the American College of Sports Medicine is 60% of your non- pregnant heart rate max."
Langer says abdominal workouts are fine - as long as they aren't too long or strenuous. "Your joints are more lax, more relaxed and so you can have more muscle injuries - that type of thing - so you want to stretch more beforehand and afterwards. But that's the main risk."
As with any prenatal decision, be sure to consult your doctor.