It's a well-established fact that blood group is decided by genotype. But, when a new child starts its journey in the womb, the mother's blood (along with it's agglutinins and agglutinogens) flows into the baby's heart. So, how does a baby (having a blood group different from that of the mother) retains it's own blood group (with it's own agglutinins and agglutinogens) instead of the mother's blood group, which moves in and out of the baby through the umbilical cord.
Do both kinds of blood flow in the baby's body constantly in such a case?
Apart from that, I know of rhesus incompatibility. But there are also incompatible blood group types. How does a baby survive agglutination in those cases?