Generally speaking, if the mother’s blood type is O and the husband’s blood type is A, B or AB, the mother and child may have ABO blood group incompatibility. The mother is sensitized by the fetal type A or B antigens and the immune antibodies produced by the fetus enter the fetal blood circulation and cause hemolysis in the newborn. About 25% of all pregnancies are ABO-mismatched, but only 2-2.5% are actually hemolyzed. However, if it does occur, it is mild, with only a few cases of kernicterus, and it does not worsen with the number of pregnancies. Why are mothers with O blood type prone to this problem (99% of pregnant women with ABO hemolysis are O)? This is because for A(B) mothers, the anti-A and anti-B antibodies in the mother’s serum are IgM, which have a large molecular weight and cannot pass through the placenta, so A(B) mothers, regardless of the husband’s ABO blood type, are less likely to develop ABO hemolysis. On the other hand, the immune anti-A or anti-B antibodies produced by a mother with O blood type, when immunized with A or B antigenic substances, are IgG, which have a small molecular weight and can enter the fetus through the placenta and cause neonatal hemolysis. Therefore, if you are a mother with O blood type and your husband has a different blood type from you, you should start the first check of anti-A(B) IgG potency at 15-16 weeks of pregnancy, when its potency is ≥1:64, the fetus may have ABO hemolytic disease, if the potency is above 1:512, it indicates a serious condition. The second check of anti-A(B) IgG potency is at 28-30 weeks of gestation and every 2-4 weeks thereafter, generally the potency is more stable after 34 weeks, if the potency does not exceed 1:64 thereafter, there should be no cause for excessive concern. If the potency does not exceed 1:64 after that, you should not be overly concerned. However, you must have a hospital checkup and consultation.