James F. Clapp III quotes:

...in early pregnancy her ability to tolerate heat stress improves by about 30 percent and in late pregnancy by at least 70 percent. Indeed, when a woman exercises at 65 percent of her maximum capacity in late pregnancy, her peak core temperature during exercise does not even get up to the level it was at rest before she became pregnant.

This means that a woman who continues regular, sustained exercise until the onsent of labor usually delivers five to seven days earlier than a woman with an active lifestyle who does not exercise regularly. What an incentive to exercise!