Look up PUPPPS it is a pregnancy rash. I developed it st 16 weeks also and still have it. 27 weeks now. Check to see if it's symptoms match yours. If so I can give you everything I do to help with it.
So I definitely asked my doctor about that whole scratching giving you worse stretch marks its not true. If you are gonna get stretch marks you will get them no matter what you do or don't do. And as far as your rash goes I would give a call to your doc's office and ask what they recommend.
i Dont think it affects baby try not to scratch youll get really bad strech marks