Some good threads here in the long past, try the search function. Also try the Women's lounge at www.mtbr.com, it's been discussed ad nauseum. Again, try the search.

It all comes down to
1. how new or old school your MD is about exercise
2. whether you are high risk or not
3. your comfort level - if ANYTHING goes wrong, would you blame yourself?
4. Your knees will be hitting your belly pretty soon anyway.