Women’s biggest concern about drinking alcohol during pregnancy is "what
about those drinks I had before I knew I was pregnant?" This is the first study I’ve seen about what affect drinking early in a pregnancy has: MedlinePlus: Prenatal drinking may boost baby’s stress response. Basically they found that two drinks a day early in pregancy cause a baby’s stress response to be greater than normal. When you are stressed your heart rate goes up and your stress hormones go up. That’s bad not only for your health but for your ability to learn.
For the record, there are lots of other things that also affect a person’s stress response.