I rode my entire pregnancies with both of my kids. With my son I rode up until my due date and had him the next day. With my daughter I rode until the final weeks and only stopped due to weather interference.
My doctor did talk to me honestly and said they are usually required to tell you that riding is dangerous because of liability insurance issues. However she told me that in reality if it's something you do often before your pregnancy it is safe to continue during pregnancy. Just don't push yourself and be on a safe horse in case of falls.
Also, if you look at my profile picture here I was about 6 months pregnant! That was my last show for the season but I did trail ride and pasture/road ride for the rest of the time.