If you choose to take dex to try to prevent virilization in the case of a girl with CAH, it needs to be started as soon as you know that you are pregnant. The genitals are formed very early in the development of the baby around 6 weeks. Then an amnio or CVS is performed to see if you have a girl or boy. If you have a boy you are weaned off the dex. If you have a girl you must wait for results to see if the baby has CAH or not. Julia