Rh incompatibility and the first pregnancy
Why is a first Rh-positive fetus carried by an Rh-negative mother usually unaffected, while later Rh-positive fetuses are at risk?
Answer C: explanation for every choice
A
The first fetus is Rh-positive and has the antigen.
B
IgG does cross the placenta, which is what causes the disease.
C
Correct. Fetal red cells usually enter the mother's circulation at delivery. In later pregnancies, her memory response produces IgG, which crosses the placenta and destroys fetal red cells.
D
Fetal antibodies don't protect against maternal anti-Rh IgG.
TakeawayRh disease: sensitization at first delivery, then IgG attacks later Rh⁺ fetuses.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID bb181.