Did you know that during pregnancy the fetus and the mother's blood mix, and that blood will stay in the mother's system for decades (this is known as 'physiologic microchimerism')? At first this was thought not to happen because the placenta acts like a biological filter preventing the blood from mixing. Later they found out this was wrong too.
After they discovered that the mother and child's blood mix, they thought this might cause problems in the mother making her more susceptible to certain types of disease. Actually, they now believe that the fetus's blood will help the mother by being able to repair tissue and possibly fight off some forms of sickness.
ref: (Khosrotehrani et al. JAMA. 2004 July 7;292:75-80)