It is impossible to say if there is a safe limit for alcohol, because its effects vary from woman to woman. What is certain, however, is that regular heavy drinking (more than 4 units of alcohol a week) can lead to mental and physical problems in your baby. To be safe, avoid alcohol in the first three months when the baby's major organs are developing. After this, it is quite safe to have an occasional glass of wine or beer with food.
Remember that you need not panic if you had a bit too much alcohol before discovering you were pregnant. There is a very small possibility that this drinking binge would have harmed your baby but going forward, remember that your baby is too special for you to take a risk. If you have any worries, do consult your midwife or doctor.