Drinking during pregnancy can be harmful to your unborn child. An article in Review Online warns:

Any woman of childbearing age is at risk of having a child with an FASD if she drinks alcohol during pregnancy. Alcohol can harm a fetus at any time, even before a woman knows she is pregnant. Many women drink early in pregnancy but stop drinking when they learn they are pregnant. Others cannot stop drinking without help. Women who have given birth to children with an FASD and continue to drink are at very high risk of having additional children with an FASD.

FASD stands for Fetal alcohol spectrum disorders and includes fetal alcohol syndrome (FAS), alcohol-related neurodevelopmental disorder (ARND), and alcohol- related birth defects (ARBD).

The Science Blog says:

Despite widespread warnings about the potential risk of drinking alcohol during pregnancy, fifteen percent of pregnant women in a newly published study said they had drunk alcohol at least once during their pregnancies. And although most of those women reported on an anonymous survey that they'd had less than one drink a week, some acknowledged drinking more than that on a regular basis, or said they'd had at least one binge of five or more drinks at once.

Yedda.com explains:

......remember that anything that you ingest goes straight into the baby's bloodstream. A baby has a very low weight, so a drink you don't even feel has a great effect on the baby. Most especially, his (her) brain. Alcoholic mothers frequently or nearly always give birth to infants with some degree of fetal alcohol syndrome.