Studies have not been able to show negative effects from moderate drinking in pregnancy. The medical community tells women not to drink because they have not been able to show that any level of drinking is "safe".

http://palatepress.com/2012/02/wine/light-drinking-during-pr...

The medical profession is positively paleolithic and incredibly patriarchal when it comes to pregnancy. There is no rational sense of weighing risks by probability, instead it's a "you can never be too safe!" attitude. My wife researched this issue heavily during pregnancy, and what really ground her gears were all the people who say "the safest option is to just not drink at all." But the fact of the matter is that these same people continued to engage in incredibly risky behavior like the wife continuing to drive during pregnancy.