I've always believed in the old maxim, "Moderation in all things," which simply means to not imbibe in anything too much. But in the case of alcohol, any is too much!
Anything that is taken into a pregnant woman's body passes through the placenta and ends up in the bloodstream of the unborn baby. So, substances like alcohol, food preservatives, antibiotics, and certain prescription drugs that have adverse side effects for the mother are going to have the same (if not worse) side effects for the unborn baby as well. We've all seen how mothers who've used heroin while expecting give birth to heroin addicts.
Several other arguments against alcohol during pregnancy include: increased risk of premature birth, stillborn birth, miscarriage, low birth weights, hyperactivity, learning disabilities, mental health problems, juvenile delinquency, congenital defects, brain damage, and stunted growth before and after birth. As for consuming a small amount during pregnancy--It's been proven that women who drink as little as a drink a week can still have babies who will suffer from a condition called Fetal Alcohol Spectrum Disorders (FASD), a variety of conditions outlined in the above paragraph.
These conditions don't necessarily happen, but they can. It seems to me that drinking alcohol during pregnancy isn't worth the risk!