Probably the most common problem would be with the Rh complex antigens. Specifically the D antigen. Presence of the D antigen makes you Rh positive. An Rh negative female giving birth to an Rh pos baby could develop an anti-D titer. This could cause serious complications with subsequent births of additional Rh positive babies. In known pregnancies of this nature, doses of Rho-gam are administered to the mother to prevent a possibiy fatal hemolytic anemia of the newborn.