Congratulations.  You need to start taking the Dex as soon as you can.  I am currently pregnant (6 months) with my third child.  First daughter has salt-losing CAH, second daughter unaffected.  I took the Dex with my second pregnancy and I had a CVS done at 11 weeks.  As soon as we got the results which showed non-CAH I stopped the Dex.  With this third pregnancy I also started the Dex early, had a CVS done at 11 weeks and results showed non-CAH again (thankfully) and so I stopped taking the Dex.  I was on the Dex for about 10 weeks both times and really the only side effect I noticed was increased appetite which resulted in increased weight.  I just couldn't eat enought food, I would never fill up!!!  I elected to have the CVS instead of an Amnio because of the earlier timing and earlier results.  I did have concerns about taking the Dex during pregnancy if the child turned out to be non-CAH so I felt it was important to get the results as soon as possible and cease the Dex if results were negative.

Best of luck with your pregnancy.