ABO and Rh Counseling for a Second Pregnancy
A couple is referred to a genetics clinic during the woman's second pregnancy, at 10 weeks. The woman is blood type A, Rh-negative; her mother was type O. The man is type B, Rh-positive; his father was type O. Their first child, now three years old, is type O and Rh-positive, and that pregnancy and delivery were uncomplicated, with a negative direct antiglobulin test on cord blood.
The woman has never received Rh immune globulin and has just had an antibody screen as part of routine prenatal care. Only the man's Rh phenotype is on record; he has not been genotyped at the Rh locus. The couple wants to know the chance that this second child will be type O, and whether this pregnancy is at risk from Rh incompatibility.
1. Assign ABO genotypes to both parents and give the probability that the second child is type O. Show the cross.
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The woman is type A, but her mother was O (OO), so she must have inherited an O allele: she is AO, not AA. The man is type B and his father was O, so he is BO.
Cross AO × BO gives four equally likely outcomes: AO (type A, 25%), BO (type B, 25%), AB (25%), OO (type O, 25%). The first child being type O confirms the genotypes, because a type O child needs an O allele from each parent - that rules out AA and BB.
The answer is 25%, with one caveat worth stating: the first child receiving an O allele does not make the next child more likely to be O. Each pregnancy is an independent draw of the same four combinations.
2. Can you state a single probability that the second child will be Rh-positive? Explain your reasoning.
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No. The mother is Rh-negative, so she is dd. The father is Rh-positive, so he is D_, but the record does not say whether he is DD or Dd - and his father's type O tells you nothing about the Rh locus, since ABO and Rh are on different chromosomes and assort independently. That detail is a distractor.
If the man is DD, every child is Dd and Rh-positive. If he is Dd, each child has a 50% chance of inheriting D. The first child being Rh-positive fits both possibilities, so it does not resolve the question either. The defensible answer is 50-100%, and the way to narrow it is to genotype the father rather than to guess.
3. Why was the first pregnancy uncomplicated, and what does the antibody screen mean for this one?
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Sensitization requires fetal red cells to enter the maternal circulation, and in a first pregnancy that happens mainly at delivery - after the baby is born and no longer exposed. So the first Rh-positive child escaped harm.
Once the mother makes anti-D (IgG), those antibodies cross the placenta in a later pregnancy, coat fetal Rh-positive red cells, and cause hemolysis, anemia and, if untreated, hydrops. Risk this time therefore depends on two separate facts: whether the fetus is Rh-positive (unknown, as above) and whether the mother is already sensitized.
A positive antibody screen now means sensitization has already occurred; immune globulin is prophylaxis and cannot undo it, so monitoring by titer and fetal assessment is the route. If she is unsensitized, anti-D at about 28 weeks and within 72 hours of delivery is the standard step.