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?
Answer
The maternal and fetal blood circulation systems are completely separate. The embryo's blood cells start developing at around week 5 gestational age (3 weeks after conception), the same way any other tissue is developed by the fetus itself. By around week 7 gestational age, a circulatory system has developed and the heart has started beating.
All nutrition goes through the placenta, which keeps the two circulatory systems separate by the so-called placental blood barrier.
Rhesus incompability is only a concern after the first birth (or miscarriage) because then the maternal and fetal blood had the opportunity to mix and lead to the mother developing antibodies.
Also, as AMR pointed out in the comments, blood group incompatibility is mediated through IgG antibodies - those don't start to be produced at all until birth and at higher levels not until six months to a year after birth. See also At what age do babies begin to synthesize their own antibodies?
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