I don't know much about Retin A, but I am fairly confident everything will be fine. Most of us have done all sorts of things that is not recommended during pregnancy prior to knowing. Many have smoked, drank, used medications not recommended etc. At that stage of pregnancy not much will impact the fetus. The important thing is to stop as soon as you find out. I understand why you are scared, but talk to your OB. I am betting he/she will tell you not to worry and be able to put your mind at ease. Congrats on your pregnancy, and I wish you a healthy happy 9 months!!
In a situation like this, I'd read the better sites (like Mayo Clinic) and ignore less-professional sites, and I would definitely talk to my doctor. There is probably a window where it is most dangerous to a developing embryo, and maybe you haven't reached it yet. Good luck.
Follow up with your OB and make sure they know about this. You will be surprised at how much a little fetus can take and still come out perfectly fine. Just follow up with your doc and he should be able to put your mind to rest.
There are multiple meds with retinoids. Like Accutane and soriatane. Most things like creams are ok if you stop using them when you find out your pregnant.
Soriatane isnt' since it's a pill that stays in your system for about 3 yrs after you stop it.
I was on it soriatane and my son TJ was conceived at the very end of the 3 yr waiting period. he didn't have any problems relating to the med.
As long as your not using it now I think the baby will be perfectly fine. :)