Treasury regulations (26 C.F.R.)--commonly referred to as Federal tax regulations-- pick up where the Internal Revenue Code (IRC) leaves off by providing the official interpretation of the IRC by the U.S. Department of the Treasury.
No. Pregnant women are not eligible to donate blood according to the American Red Cross, which also advises waiting at least six weeks after giving birth. Although the amount of blood in your body increases by about 50 percent during pregnancy, you and your growing baby need it for optimum health and nutrition.
In fact, if you are already pregnant when you take Plan B then you will still be pregnant afterward, because it only works to prevent implantation before pregnancy, but has no affect if you are already pregnant. So, you should not expect to see bleeding right away unless you were due for your period anyway.