You may not be able to keep up that five-mile run when you're six months' pregnant, and that's totally fine. Hormonal changes during pregnancy make you more flexible (and therefore, prone to injury) and your growing belly can throw off your center of balance, making your standby workouts more difficult. You may need to cut your workouts a bit short (take a 30-minute walk instead of an hour), or decrease your speed or resistance. If you take classes, always be sure to let the instructor know that you're pregnant so he can help you adjust the moves accordingly.
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