Based on a websearch, it appears that alcohol concentration in breast milk is similar to that in blood, so for a drink or two it would be about 0.05%. However, it appears that a fetus gets most of its alcohol directly from the mother's blood through the placenta. Apparently alcohol consumption by the mother inhibits milk secretion. Also, alcohol in the baby inhibits the sucking reflex to some extent.

From Department of Human Services Victoria, Australia, Australian Drug Foundation: While the effects of drinking alcohol on breast feeding are unclear, it is known that alcohol is excreted into breast milk- It may be that having one drink occasionally is safe, but in general it is better to avoid using alcohol as much as possible.

From Journal of the Diabetic Association of India: Education Section. 37(4). Oct-Dec 1997. P. 61-63The concentration of alcohol in a mother's milk is about the same as that in her blood.