Exposure to alcohol, cigarettes, or drugs very early in pregnancy is unlikely to cause birth defects. It may increase your risk of miscarriage but as long as that exposure is stopped early it is very unlikely to cause long term problems for the baby.
There is an “all or nothing” phenomenon in the early first trimester. For the most part if an exposure is very early in pregnancy it will either stop development and a miscarriage will happen or the bay will be fine. It is important to stop using drugs, smoking, and drinking alcohol as soon as you find out you are pregnant.
Since very early exposure is so unlikely to cause birth defects, if the baby is doing well by the beginning of the second trimester, you do not have to worry about problems at birth.