According to the research, once the third trimester starts, you should not venture far into anaerobic intensities for extended periods of time because you are at risk of fetal hypoxia. In English- don't work too hard for too long... going up a big hill during an easy ride is ok, but allow yourself to recover afterwards so that you can keep the oxygen & blood supplied to both of you!

Also, you are more likely to lose your balance as your center of gravity is "modified."

That being said, I've heard stories of women running pretty fast marathons within days of going into labor with no ill effects to the baby. I've also had pregnant students in weight training classes at the university where I teach that lifted pretty heavy the whole semester and had their babies soon after. It was the 2nd child for one & she said that being stronger made a HUGE difference in being able to push that baby out.
I think you should do whatever you can as long as you can!