I am nearly 26 weeks now and have just about stopped riding. I only ride on roads/paths anyway, so not quite as bumpy as yours (mind you, with traffic it's probably more dangerous).

I asked my OB about it when I realised I was starting to get puffed on the hills, and my heart rate was going over 140. She told me that was long as it was exercise I was used to doing from before I was pregnant, and as long as I remained comfortable and not getting too puffed, it would be fine to keep exercising at that rate.

Lately, I find that I have gotten too nervous about the risk of falling or being knocked off my bike. It's scary not so much because hurting the baby is likely but because worrying that you might have hurt the baby is awful (when I fell off my desk I worried for hours afterwards - not worth the stress). So I'm hardly riding at all now. But your experience may vary from this.