Get a lunge whip that you can snap, a lot of times that is enough to get her to respect it, and in the future you wont always have to be so forceful. If it doesnt work, you may need to make contact. Some horses need the reminder that the whip can, and will if necessary, hit them. Not that you have to beat her, but sometimes even letting it "tickle" her will get them moving.
Another thing that seems to work is attaching a plastic bag to the end of a whip and shaking it. Usually they have no problem understanding to move forward with that chasing them.
I am pregnant, too... when are you due? I'm pregnant with my second, and continued to ride with my first until I was 8 months pregnant. I'll probably stop a little earlier this time...
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."