I think the alcohol guidance is not medical at all, but social. Pregnancy is the one time in your life when suddenly it becomes acceptable to tell you what to do, what to eat, what to wear, how to act, etc., and the world seizes on it - suddenly everyone turns into the Daily Mail! The idea that one glass of wine can call FAS is absolutely outrageous - not just wrong, but vicious - whoever is publishing this rubbish should be strung up, because they are not doing it to be helpful to pregnant women, but to make them feel guilty.
A friend of mine who is a paediatrician believes that there is a strong genetic component in FAS, as it seems to run in families, and also, that you have to try really really really hard to give a baby FAS - like drinking a bottle of gin a day, which apparantly Anne Robinson did when she was pregant and still had a healthy child.
I found that my body actually worked - it told me when I'd had enough coffee, wine etc.- funnily enough I didn't need the pregnancy police directing my every move - and I'm sure that unless you're an addict, most women's bodies do the job just fine themselves, although they will all do it differently, because we are all different and have different threshholds etc.