My doctor (high-risk perinatologist) prescribed Colace because I was suffering from constipation so much. It was like a lifesaver for me! Really helped me get through this pregnancy because I suffer from terrible constipation.
Colace is Pregnancy Category C...but my OB says it is safe. In fact, she gave me samples of a prescription prenatal vitamin that has 55 mg of docusate sodium (aka Colace) in them.
You can take over the counter fiber pills and stool softeners. I think the prune juice is the fastest acting thing you can take, but the pills should kick in before morning.
You could try the juice mixed with sprite so the taste isn't so bad. Feel better soon!