I don’t have PERSONAL experience with this, but here are my two cents:

Why would this baby need to undergo an ACTH stimulation test anyway?  If the mother has been taking dex during pregnancy, then---presumably---it has already been determined (through CVS or amnio) whether the baby has CAH.  So, there should be no real need to do a stim test.

I know there are some mothers on this mb who have taken dex during pregnancy and given birth to a CAH baby, so it would be interesting to hear from them directly.  But it is my impression that those children were NOT given ACTH stim tests at birth, just simple blood draws. 

Also, A.  to comment on your original question about whether dex would affect the baby’s blood levels:  As Megan said, the answer is Yes.  However, I would like to add that that is really the whole point of taking the dex....you WANT to affect the baby’s levels!  If you are carrying a girl with CAH, the dex lowers her androgen levels, so that severe virilization can be prevented.  If the dex did not affect the baby’s levels, there would be no reason to take the dex.