One of the ways i noticed that i was pregnant (like the day before i got my pos HPT), was my heart rate was about 40 points higher when i was on the cross trainer than it usually is.

I also felt the exercise a lot more, even though i suspected i was preg and wasn't working out so much.

My obs did tell me that when i do exercise, to keep my heart rate under 140.. although he says he hasn't really seen anything to say it will do much if you let it go over (within a reasonable amount).