It's unlikely that an occasional drink before you realized you were pregnant will harm your baby. But the baby's brain and other organs begin developing around the third week of pregnancy, so they could be affected by alcohol in these early weeks. The patterns of drinking that place a baby at greatest risk for fetal alcohol spectrum disorders (FASDs) are binge drinking and drinking seven or more drinks per week. However, FASDs can and do occur in babies of women who drink less.
Because no amount of alcohol has been proven safe during pregnancy, a woman should stop drinking immediately if she even suspects she could be pregnant. And she should not drink alcohol if she is trying to become pregnant.