Alcohol

Found in wine, beer and liquors, is a toxic substance that is quickly transferred from the mother's bloodstream to the baby's.

Because all the organs of the fetus are forming, they are particularly vulnerable to toxic substances, especially during the first three months of pregnancy. In the best interests of the baby, a pregnant woman should change certain habits from the very beginning of the pregnancy (e.g. stop drinking alcohol, avoid all other toxic substances, and watch what she eats).
To date, researchers have not been able to determine the exact amount of alcohol that is "safe" for the development of the fetus. We do know, however, that the risks of miscarriage, birth defects, growth retardation and mental disorders increase the more the mother drinks, and the more frequently she drinks.

If you just learned that you're pregnant, but have had a few drinks recently, you don't need to be overly concerned about your baby. A pregnant woman who is thinking about drinking, even moderately, should consult her doctor so that she can make an educated decision.

The Risk

Heavy drinking, however, can gravely affect the fetus. A pregnant woman who frequently drinks heavily is more likely to give birth to a child with a specific condition known as fetal alcohol syndrome, or FAS. FAS includes a number of problems such as growth retardation, mental disorders, an abnormally small head or other facial and skeletal abnormalities, and malformations of the heart. Occasional heavy drinking also increases the risk of similar problems.

Any woman who can't stop drinking should discuss the problem with her doctor or another medical professional before even thinking about getting pregnant, and then throughout the pregnancy, to ensure she gets the support she needs to abstain.

Overall Health

Alcohol becomes increasingly toxic the more you drink and the more frequently you drink. But keep in mind that alcohol is never the only factor involved in the development of the baby. The parents' basic health, their medical history, their lifestyle, the mother's diet, outside pollutants, tobacco and drug use during pregnancy, and other socioeconomic factors all have an impact. Even under ideal conditions, no woman can completely control the outcome of her pregnancy. 
If it's important to you on particular occasions, like an anniversary or birthday, you can safely drink one or two glasses of wine while eating, especially at the end of your pregnancy. But under no circumstances does anyone have the right to force you to drink. And nobody should make a pregnant woman feel guilty for drinking moderately.

Breastfeeding

A woman who is breast-feeding must think about her baby's well-being, before choosing to drink. A nursing mother should eat a balanced diet, get plenty of rest, and be careful about her alcohol consumption. You should not drink while actually nursing, because the baby will absorb the same amount of alcohol as you do, as it nurses. If you combine breast-feeding with bottle-feeding, you can drink moderately on occasion (no more than the equivalent of two drinks), and give the baby a bottle at those times. Be aware that it takes a minimum of two hours for your body to eliminate the alcohol in one drink.