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?

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    $\begingroup$ maternal circulatory system is separated from fetus in the womb $\endgroup$ – AliceD Feb 18 '15 at 4:41
  • $\begingroup$ What happens before then? And answer my second question if you can. $\endgroup$ – elvarox Feb 18 '15 at 5:59
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    $\begingroup$ Rh incompatibility is due to fetal blood which escapes into the maternal circulation during delivery. It does not occur always and depends on labor conditions. There is absolutely no mixing of fetal and maternal blood otherwise. $\endgroup$ – One Face Feb 18 '15 at 7:47
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    $\begingroup$ Note also that Rh-incompatibility is most often an issue with second or later pregnancies, after antibodies have been formed during earlier pregnancies, see e.g. nhlbi.nih.gov/health/health-topics/topics/rh for more. @CRags $\endgroup$ – fileunderwater Feb 18 '15 at 9:45
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    $\begingroup$ @CRags I suspected that, and my comment was mostly directed to the OP. $\endgroup$ – fileunderwater Feb 18 '15 at 10:18

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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    $\begingroup$ I wrote the answer before reading that there are already comments answering the question :-/ Still, this shouldn't be in the list of unanswered questions. $\endgroup$ – YviDe Dec 23 '15 at 12:13
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    $\begingroup$ You may want to add that the fetus does not develop the ability to produce antibodies until around six-months after birth, so even if the mother's RBC's did circulate in fetus, they wouldn't be producing antibodies against them and by the time they started to, all the maternal RBCs would have degraded. Fetal blood crossing to the mother would be far more relevant as the mother would make antibodies against it which you mention with your last paragraph about first birth or miscarriage. $\endgroup$ – AMR Dec 23 '15 at 18:20
  • $\begingroup$ @AMR good point, thanks! Though, I think six months is too late - isn't it way earlier? $\endgroup$ – YviDe Dec 23 '15 at 18:21
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    $\begingroup$ Class switching takes a while. "After birth, the production of IgM starts almost immediately; the production of IgG, however does not begin for about 6 months, during which time the total level of IgG falls as the maternally acquired IgG is catabolized." - Janeway's Immunobiology 8th Edition. $\endgroup$ – AMR Dec 23 '15 at 21:21

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