No, dex doesn’t prevent CAH, it merely prevents (or in some cases partially prevents) the virilization of the external genitalia of the females. 

The dex begins as soon as the pregnancy is confirmed (hopefully by the 5th week), and is continued until the results of the CVS are in (about the 12th week).  If the mother chooses amniocentesis instead of CVS the test will be about the 16th week instead.  So the mother would be ingesting the Dex for approximately 7 to 11 weeks (unless the baby is a CAH girl).  If the baby is a boy (CAH or non) and if the baby is a non CAH girl, then the dex is generally discontinued.  Some mothers of CAH males decide to continue to take the dex for the remainder of the pregnancy to make up for the lack of cortisol production in the infant so that birth will not be as stressful on the child.

At this present time there is nothing that can "prevent" this birth defect (CAH). 

Odds for parents who each carry the gene are 1 in 4 per child for each that they have (25% chance for every pregnancy they have).  Odds for parents with one parent who is affected (a CAH mom or dad and the opposite parent is the carrier) is one in 2 or 50% chance for every pregnancy.

Hope that answered your original question.

Take Care,

Adina