tl;dr - It takes more than a moderate alcohol consumption to cause serious alcohol related birth defects at a high rate in a given population.
I can't give a well sourced historical answer but I can offer a biological one. Simply put, the human body is more resilient than we give it credit for. Alcohol is not as likely to cause defects as most teratogens(birth defect causing chemicals). Sure, large amounts of alcohol in the first trimester can cause FAS (fetal alcohol syndrome) but Its not a zero tolerance game like with really potent teratogens (read: Thalidomide) If I remember my toxicology properly, the more common worry with alcohol during pregnancy is low birth weight and alcohol withdrawal after after birth.
I would guess that you would need really detailed medical records to differentiate alcohol related complications in pregnancy with other common nutritional and environmental ones like folic acid deficiency and environmental lead poisoning. Hopefully there is a medical historian here who can shed light on the when, where, and how of discovering and studying birth defects caused by diet and environment.
Hum, how to deal with this question.
First, I'm not sure what 'in the past' means, nor where in the world you are referring to. Without either of these, the question is almost unanswerable.
However, for the sake of trying to get at the underlying point, if I look at the average /r/askhistorians question about heavy drinking in the past I would say this reflects common mythology about the medieval period. In which case I refer you to the following discussions which undermine the premise that 'people drank so much alcohol in the past', one usually argued on the basis that fresh water was not available.
Fresh water vs alcohol in the middle ages: How did people (esp. European townsmen) get their fresh water? and How drunk were the people of medieval Europe? (see further discussion here)
And for shits and giggles, this question came up a few weeks ago in which someone addressed the hypothetical scenario of excess drinking and evaluated the possible incidence of FAS based on modern understanding of occurrence of it: If people drank beer all day in the Middle Ages in Europe, why wasn't everyone born with fetal alcohol syndrome?