A difficult situation! A study found that pregnancy does tend to worsen FM symptoms unfortunately. The below website gives FM patients some suggestions of how to get through pregnancy. It focuses on alternative methods such as meditation, massage, heat and pool therapy. These may sound kind of lightweight but studies have shown that these types of practices - which turn down the stress response system (and reduce levels of pain inducing chemicals) - can be quite helpful in FM. You should also be doing your best to get good sleep - as poor sleep can cause FM flares as well.
Perhaps if these therapies work for you now you'll be able to reduce your prescription drug intake later as well.
http://www.fibromyalgia-symptoms.org/fibromyalgia_pregnancy.html
Good luck!