It's good that you are concerned. It's reasonable for your sister to lower her exercise intensity during her pregnancy for several reasons:

1. When you exercise, your body temperature rises as a result of increased blood volume. Externally your body temperature rises, and also internally. Internally, your body temperature is several degrees higher than externally because there isn't a cooling mechanism on the inside that works like the air on your skin outside. Therefore, the fetus temperature can rise to unsafe levels.

2. During any extreme physical activity like exercise, the blood flow shifts from other organ functions to your working muscles, and to the skin to keep you cool, which means the blood flow to the fetus is diverted temporarily -- which is the same reason why when you exercise shortly after a meal, digestion stops or slows down. After your exercise you may feel like your food is still sitting in your stomach. That's because it is!

The conclusion to all of this is that according to the American College of Obstretics and Gynecology, you should not let your heart rate exceed 140 beats per minute. If her doctor told her 150 bpm, it's probably because she's in good physical condition. For someone who is normally sedentary, 140 bpm may be too high. Two hundred bpm is definitely not a safe range during any stage of pregnancy.

It's also recommended that you pay close attention to the duration of your exercise sessions. Forty minutes is good. If she really likes working out, then twice a day for 40 minutes is better than once a day for one hour. She should make sure there is plenty of ventilation to allow her body to cool down properly. Aerobics classes in poorly ventilated rooms are not advisable. Your sister should also drink plenty of water to replace what she's losing during exercise. I would reccommend twice as much as normal if she continues to work out often.

And, MOST IMPORTANT, take the opportunity to remind your sister that the nurturing of a growing baby in your womb takes special care and attention. This is a time to only be concerned with fitness maintenance, and what you need to do in order to allow for a healthy child to grow in your womb. Your concerns are valid, but you may be pleasantly surprised to know that in spite of all of the guidelines that I just gave you, your body will force you to regulate your fitness activities as a natural response to pregnancy. All she needs to do is listen very closely to what her body and baby wants, and not to continue on automatic pilot doing all the things she's normally used to doing.

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