Newborn Blood Groups and a Suspected Nursery Swap
At a district hospital in Nepal, two newborns were placed in the same nursery within 20 minutes of birth. Couple 1: mother group A, Rh-negative; father group O, Rh-positive. Couple 2: mother group AB, Rh-negative; father group A, Rh-positive. Baby X, handed to couple 1, is group B, Rh-positive. Baby Y, handed to couple 2, is group O, Rh-negative. No wristband error was recorded, but the nurse admits the babies were briefly unlabelled during a shift change. The hospital has only ABO and Rh antisera and no on-site DNA testing. Both families are distressed; one father has threatened legal action. The medical officer must decide whether to declare a probable swap, refer for DNA profiling, or reassure the families that the serology is inconclusive. Cord blood from all four parents and both babies is still stored. The hospital ethics committee wants the reasoning documented before any disclosure.
1. Determine whether either couple can be excluded as the biological parents of the baby they received. Show the possible parental gametes and offspring genotypes, not just the final answer.
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Couple 1 (mother A, Rh-negative; father O, Rh-positive)
- ABO: mother is IAIA or IAi; father is ii. Gametes: mother IA or i; father i only. Offspring can be IAi (A) or ii (O). No parent carries IB, so a B child is impossible.
- Rh: mother rr; father is R_ (RR or Rr). A Rh-negative child (rr) is possible only if the father is Rr, with probability 1/2. A Rh-positive child is also possible.
- Baby X is B, Rh-positive: excluded from couple 1 because B is impossible.
- Baby Y is O, Rh-negative: compatible with couple 1 (O possible; Rh-negative possible if the father is Rr).
Couple 2 (mother AB, Rh-negative; father A, Rh-positive)
- ABO: mother IAIB; father IAIA or IAi. The mother always contributes IA or IB, never i. Offspring can be A, B, or AB; O (ii) requires i from both parents and is impossible.
- Baby Y is O, Rh-negative: excluded from couple 2 by ABO alone.
- Baby X is B, Rh-positive: compatible if the mother contributes IB and the father contributes i (father must be IAi); Rh-positive is possible if the father is Rr.
Conclusion: the serology indicates a probable swap: X fits couple 2 and Y fits couple 1. This is a one-way exclusion argument; it does not by itself prove parentage.
2. The administrator says ABO groups are inherited, so ABO typing alone can 'prove' parentage. Evaluate this claim and state what evidence would make the conclusion defensible.
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ABO inheritance can exclude a parent, but it cannot prove parentage. A compatible ABO type is common in the population, so many unrelated people could be compatible. In this case, the strongest statement is exclusion: B from couple 1 is impossible, and O from couple 2 is impossible. Those exclusions are valid only if the phenotypes are correctly typed and inheritance is simple. Caveats include the Bombay phenotype (hh), which can mask A and B antigens and type as O; cis-AB alleles; chimerism; recent transfusion; clerical or labelling error; and rare mutations. Additional evidence: Rh and MN typing add little because these types are common. HLA and, best, DNA STR profiling of multiple loci from both parents and both babies, with a calculated parentage index, would make the conclusion defensible. Chain of custody and repeat sampling are essential for legal use. Thus ABO alone justifies referral for DNA testing, not a formal declaration.
3. Explain why Rh status adds little to the exclusion here, but matters clinically for the mothers.
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Rh adds little to exclusion because the Rh-positive fathers could be RR or Rr. A Rh-negative mother (rr) can have a Rh-positive child with either father, and a Rh-negative child if the father is Rr. Thus Rh does not exclude either couple; ABO does. Clinically, Rh matters because both mothers are Rh-negative. If the swap is confirmed, the mother who is biologically related to Baby X (Rh-positive) is at risk of anti-D alloimmunization if fetal-maternal bleeding occurred; she should receive anti-D immunoglobulin within 72 hours. The mother of Baby Y (Rh-negative) is not at risk from that baby. Until the swap is resolved, the hospital should treat both Rh-negative mothers as potentially exposed and follow obstetric protocol.