Where does fetal blood come from? Well the fetus is responsible for producing its on blood just like it forms its own bones, skin, etc. The blood type is determined by the parents. Type A and B blood are dominant so if the gene is present this blood type will show up. Since type O is recessive, for a fetus to have this type both parents must have the type O gene. Since mother’s blood and fetal blood never mix; if there is some kind of leakage of fetal blood, antibodies will be released, which can then be dangerous for future pregnancies. This is because if the mother is to get pregnant again and the baby happens to have a different blood type the antibodies that were produced during the first pregnancy could potentially hurt the baby if they happen to cross the umbilical cord. Pregnancy is complex, man.