Nai the arms above head thing is a blood pressure precaution - if it's already low doing that can make you faint.

There have been studies which found that vigorous exercise (at 90-100% of your max heart rate for 10 or more minutes, the sort of things competative athletes do) can increase the likelihood of miscarriage but the studies themselves had NO women with BMI over 20 in them, and low BMI is also associated with increased miscarriage risks, so it's hard to know if the exercise was to blame.

Your body will work hard to protect your baby while you exercise. It might feel harder, you will be warm faster and sweat more easily and more profusely. Your body will keep your baby safe unless you do explosive anaerobic training (like sprinting). Even if you DO do that sort of thing, you're far far more likely to faint than miscarry.

There are women in Africa who walk 10 miles just to get water every day of their pregnancy, and they can do it, why? Because their body is used to it. So in general you can continue with what your body is used to and stick to the same effort level. As you get bigger your perception of effort will change - weights in pump that used to feel ok will be too heavy, you'll be exhausted after shorter runs or less time on the bike. Listen to your body.

Pregnancy is not a time to INCREASE fitness, but you can absolutely maintain it safely so long as you listen to your body. FWIW when i ran my half marathon last year a woman came in an HOUR before me, 7 months pregnant.

Bx