Hello, and I hope I can help you today. It is true that alcohol consumption is not good for a developing fetus; it affects the growth of the brain and daily heavy drinkers are at risk for certain types of birth defects. However, if you were only a day late with your period, you are at most 5 weeks pregnant. At this time, there is no fetus yet and only the gestational sac has formed. Therefore, there is no developing brain yet that can be harmed by the alcohol you already drank. So you do not need to worry at this point about the alcohol you consumed before you knew you were pregnant. Many women find out they are pregnant after a history of alcohol consumption and have completely normal pregnancies as long as they stop drinking. I hope I was able to adequately answer your question today and that this information was reassuring. Best wishes for the remainder of the pregnancy, Dr. Brown