We can't tell at which stage of pregnancy it causes birth defects, nor at what quantity. Small amounts at the wrong time can cause it, whereas large amounts at other times may have no effect. And people are notoriously terrible at self-reporting. We can't perform these experiments properly because, y'know, ethics, so you can't know that drinking in moderation will not cause birth defects (plus everyone has a different definition of "moderate"). As a result, the CDC recommends absolutely no alcohol consumption whatsoever during pregnancy.

http://www.cdc.gov/vitalsigns/fasd/

FAS actively promotes inequality in all areas of life by causing brain damage to children in demographics more likely to be consuming alcohol, and creates a cycle of failure that can't be resolved in that generation. It makes life harder for the parents, who have misbehaving children who need extra attention and can't perform as well in school. It makes life harder for the victims, who live life with brain damage and difficulty with all varieties of thought. And it makes life harder for their kids, whose parents are likely more stressed out and need more time to resolve problems. Not to mention the societal cost to all those around them, and the opportunity costs from lost productivity. Anyone in favor of social justice and equality should be against starting people off with brain damage.