as far as i know, the major parts of genitalia structures are formed between 6th and 7th week. that is why it is imprtant to take dex before 7th week.

and once you pass the first trimester, the recommendation is to reduce the dosage becasue the major parts are already form by then.

the other thing is...if you stop taking dex after the first trimester, the affected baby girl might be born with larger clitoris but might not need a vaginal surgery.

and afterall you might have a baby girl with CAH with minimal virilization with Dex intake during pregnancy

so nothing is certain.

that is what i know, hope it helps