Your baby's major organs begin to develop around the third week of pregnancy, giving you a tiny cushion. While it is best to discontinue drinking and other behaviors known to cause potential harm to your baby before even attempting to get pregnant, sometimes it doesn't work that way.
There is some preliminary research that says even a few drinks this early may cause fetal cell death. This may or may not mean trouble for the baby or pregnancy.
One piece of good news is that early pregnancy tests are now more sensitive. This means that you can discover your pregnancy sooner and take corrective measures to stop unhealthy behaviors like drinking and smoking. This gives your baby the best chance at a healthy start.
Be sure to be honest with your doctor or midwife at your prenatal visits. Explain to them what happened before you knew you were pregnant. They will then have a chance to do extra testing if called for, reassure you or advise you in helping to adopt a healthy lifestyle. Don't be embarrassed or worried - they have experience dealing with this on a daily basis.