We generally want to be safe during the 1st trimester. However it also depends on what you have been doing prior to your pregnancy. I would say doing 50% less (in term of frequency, intensity, duration) than what you have been doing shouldn't be a problem provided that your pregnancy is not high risk and you are not experiencing any of the below symptoms: spotting, cramp, short of breath, high blood pressure and any discomfort that you are not sure of the reason. Below are some example of exercise modification: - If you used to climb 5 flight of stairs as an ex, you can try out only 2 flight of stairs, and do it slower than how you used to do. - Reduce the duration you hold in plank by half. - Do body weight squat instead of weighted squat. - Do wall push-ups instead of regular push-ups.