While your doctor's advice should be heeded because he or she know your particular circumstance, I know of no evidence that moderate exercise causes miscarriage. I would consider four miles a day three to four days a week moderate. If you run seven days a week, you may not need that much exercise and sould consider opting for four to five days.

During pregnancy, you should be able to do what you did before pregnancy as long as it poses no risk to the fetus. Be careful to stay well hydrated, cool and consume enough carbs to compensate for your exercise. Exercisers are more prone to hypoglycemia and a lack of glucose can inhibit baby's growth and development.

Also, you may want to look into water aerobics or videos like Kathy Smith's Pregnancy Workout to learn exercises for your changing body. Some muscles will need special attention during the second half of pregnancy to prevent discomfort and long-term posture problems. Be sure to stretch your lower back muscles, hip flexors and pecs. Also be sure you strengthen your upper back and abs (avoid crunches after the first trimester). Good luck!

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