Fetal Blood Blog

Fetal blood is created by the fetus itself, the fetus makes it's own blood. The fetus's blood type is determined by both of it's parents. All the blood types are A, B, AB, and O. The blood types A and B are both dominant, so if the fetus receives an A type from one parent and a B type from the other, the baby will have type AB blood. Since type O blood is recessive, both parents would have to give the baby an O type gene for it to have type O blood. If the baby has a different blood type from its mother, than the mothers body will make antibodies to the baby's blood type. Usually this isn't a problem during the first pregnancy. If during her other pregnancies the baby has a different blood type from its mother, the antibodies that her body created during the first pregnancy can cross the umbilical cord into the baby and hurt the baby. However there are shots that can be given to the mother during her first pregnancy and what it does is suck up all the baby's blood cells so that the mother's body does not make antibodies to it.

 http://www.newton.dep.anl.gov/askasci/mole00/mole00338.htm