You haven't lost all the baby weight yet, but chasing your little sprinter across playgrounds and up stairs is getting you into great shape. Isn't that enough to protect your heart? Not quite. Exercise is good for the heart, but if you're lugging around extra fat, it's not enough, finds a 24-year-long study of women published in The New England Journal of Medicine. That's because keeping your weight within normal limits is a powerful factor in preventing heart disease. Here's what else the study found: If you're of normal weight and exercise just 30 minutes a day, you have the lowest risk of anyone. But if you get only an hour or two a week, your risk goes up by 50 percent. If you're overweight and sedentary, your risk is 250 percent higher than that of a normal-weight active woman. Working out brings that number down: If you exercise regularly, your risk is 50 percent higher. Bottom line: For a healthier heart, aim for at least half an hour of moderate exercise (like brisk walking, biking, or swimming) a day. But don't toss out that scale.