I was exercising for about an hour at a time about 4 times a week before conceiving as well as doing weights (mainly through pump classes). After 8 weeks of this we got pregnant and from about 6 weeks gestation I just monitored fluid intake and heart rate.

The recommendations are to remain hydrated, don't overheat and keep HR less than 140bpm. Cycling, running, walking, swimming etc are all fine if you were doing them before, otherwise check before starting. Weights are also fine but the focus should change to less weight and more repetition. Abs are fine until 16weeks and after that they don't suggest lying flat on your back. I continued to do pilates with the half swiss ball. Check with the instructor as to the type of core work you can do to strengthen the abs and back to prevent back pain without doing crunches.

I have a friend who continued to run, do weights etc until the day she gave birth (she went to the gym the day she went into labour!!!).

Check with your gym instructor to get a program specifically designed for your stage of pregnancy. Check with your OB to make sure they are happy with you to continue medically.

Do what you feel comfortable with and trust your body to tell you when to slow down. Ultimately, a fit healthy mother often has a better pregnancy and labour experience.