How long do I need to walk per day while pregnant to maintain my weight?

30 minutes. Half an hour of fast walking daily is sufficient for most pregnancies (treadmill just as good, if not better due to cushioning of step vibrations). Your obstetrician will tell you if you need less or more exercise depending on other conditions such as poor fetal growth, diabetes mellitus, other maternal diseases. Make sure you remain well hydrated and not short of breath!