First of all, let me say congratulations on your pregnancy, and I am glad to hear that you are making helth conscious choices for you and your baby during this pregnancy. You ask a number of good questions regarding vitamin B12 and the association with alcohol; many of which I will attempt to answer for you, however, I am going to recommend that you ask the same questions to your OB/GYN as there are other causes of macrocystic anemia
(such as a folate deficiency) which can potentially be bad for you baby, which your OB is going to want to sort out with you. You are correct in saying that vitamin B12 deficiencies are more common in those that drink a lot of alcohol. There is evidence to suggest that alcohol does in fact decrease the absorption of vitamin B12 (perhaps by altering intrinsic factor). Also, there is evidence that people that drink a lot of alcohol don't typically have the most well balanced diets, and it is this dietary deficiency that leads to B12 deficiencies in high alcohol drinkers. In particular there are usually fat deficiencies in the diets so the fat soluble vitamins (A, D, E, K) are also typically low. Likely the correct answer lies in some combination of the two theories. Generally speaking, the short answer to your question about B12 deficiencies is that it is possible that your 'stores' had been depleted during your drinking times and they will need to be restored. This can be done by eating a well balanced diet, in particular eating foods such as meat, fish, dairy products, and eggs. Also I would recommend taking a pre-natal vitamin which has both folate and B12 in it during the beginning weeks of your pregnancy. I hope that this in part answers your questions, however it is very important that you speak to your OB/GYN who can examine you. Best of luck.