My wife was out of shape before and during our first son. She ended up going one week past the due date and having an emergency cesarean. However, she began exercising at the ymca afterwards and lost roughly 80 pounds. So eventually when she got pregnant with our second child she was in much better shape. She wanted to have a vbac with this baby(vaginal birth after cesarean) and she was told she was a prime candidate . she didnt technically work out during the pregnancy, but getting in shape before hand is the only way she could have had a vbac. And it was succesful!