From CNN Correspondent Dr. Steve Salvatore

NEW YORK (CNN) -- Exercise during pregnancy used to be considered dangerous. But new research shows that exercise is not only safe -- it's recommended.

Certified master trainer Heather Nutting hasn't stopped her daily workouts despite being seven months pregnant.

"I still do aerobics; I still do weight training; I still do ballet, and I definitely stretch; but it's all careful," she said.

Certain types of exercise such as high-impact aerobics and weight training used to be considered too risky, and doctors put restrictions on the intensity and duration of training. But experts now believe such restrictions are unnecessary.

"We found that low to moderate exercise was safe," said Maureen Hatch of Mount Sinai School of Medicine. "That is, it reduced the risk of pre-term delivery."

A new study from the school found that exercise may offer some extra benefits.

"Heavy exercisers -- and by heavy I mean something on the order of three to four hours of regular exercise such as swimming or aerobics every week -- those women were more likely to give birth right at 40 weeks of gestation," Hatch said.

Researchers also found exercise is safe for women with a history of miscarriage and early birth.

"We did not find that exercise increased the risk of pregnancy-induced hypertension or aggravated chronic hypertension," Hatch said. "In fact, several studies have shown that exercise may be a prescription for women with diabetes."

As with any exercise program, experts advise a dose of common sense: Don't take on too much too fast, increase your calorie intake, and take cues from your body.

Experts also recommend avoiding exercises that require balance, because a fall could be dangerous for the mother and baby.