How can I know if I harmed my baby by drinking before I knew I was pregnant?

Your 20 week sonogram may be able to show up some physical markers that might indicate FAS. But to be honest, this has happened to a lot of women who went on to have a healthy child. At 5 weeks, your baby is still not attached to your blood supply and relies on a yolk sac full of nutrients and oxygenated blood which shrinks over time until the placenta begins to take over. As long as you stop now, now that you're aware of this pregnancy, there's a very strong likelihood that your baby won't suffer any long term ill effects from your drinking.