Alcohol in Pregnancy
The consensus from the medical world is that "no amount of alcohol in pregnancy is safe". This doesn't mean that a few drinks before you knew you were pregnant is definitely a problem, but there have been no studies showing that some amount had absolutely no effect. The best thing to do is to have a healthy, whole foods diet and abstain from alcohol when trying to conceive and during pregnancy. That way, no matter the outcome, you know you did everything you could to nourish your baby.
Large amounts of alcohol while a woman is pregnant can lead to Fetal Alcohol Syndrome in her baby. This causes neurological damage that is permanent. It's especially harmful to do a lot of drinking early on in the pregnancy, when the baby is still developing. Damage at this stage could be severe. If heavy drinking is continued through the pregnancy the baby can have withdrawal after birth, which is obviously not what you want to be dealing with.
Alcohol in Breastfeeding
Breastfeeding gets a little more complicated when it comes to alcohol. The good news is, after 9 months of no margritas you can have one! The caution is that the alcohol could still have a negative effect on your baby, so there are some guidelines.
Think of your breastmilk just like your blood. After all, milk is made frm the substances carried in your blood. We all understand that the alcohol level in your blood goes up as you drink and then gradually goes down when you stop. The same happens with your milk. The alcohol is cleared from your milk when you stop drinking. The rule of thumb is to wait one hour per drink to nurse your baby. If you're still feeling the effects of the alcohol then you should probably wait to nurse.
Some people have recommended beer as a galactagouge, or a food that increases milk supply. There is little evidence that this works, because beer is not made the same way as it used to when it was recommended. It was only the stout beers that had this positive effect. You can read more about the history in this article.
Other kinds of alcohol, especially large amounts, have a negative effect on milk production. They reduce production of oxytocin, which is necessary for milk let-down. Because of this, it's probably a good idea to keep from drinking until your milk supply is well established and your baby is gaining weight well, and even then keep from overindulging.