HY lecture notes:
Two main types of hemolytic disease of the newborn:
- Rhesus factor (-) type
- ABO type
Rh (-) type:
Always a mother in second pregnancy or later. Never in first pregnancy.
Mom who is Rh (-) does not have Rh antigen on her RBCs, nor does she have antibodies against Rh antigen.
If the father of the fetus is Rh (+), there is a high chance the fetus is also Rh (+).
During parturition, if there is mixing of the fetal and maternal blood (i.e., during traumatic labor), the mother’s immune system will recognize the Rh antigen on the fetal RBCs as foreign, and in turn will develop IgM (and then later IgG) antibodies against it.
In subsequent pregnancies, the mother’s IgG against Rh factor will cross the placenta and attack the fetal RBCs that have Rh on the surface (assuming Rh + fetus).
This results in hemolysis in the fetus of varying severities. The fetus may become severely anemic in utero, requiring transuterine blood transfusions, or may experience pathologic jaundice following birth.
Maternal Rh status must be assessed during first pregnancy.
For second pregnancies-onward:
At 28 weeks gestation + prior to labor, the Rh (-) mom will receive RhoGAM to prevent her immune system from creating antibodies against Rh just in case there is mixing of her blood with the fetus’s. RhoGAM refers to Rh immunoglobulin that will bind to and mop up any antigen that has entered her circulation.
RhoGAM is not just given at 28 weeks + at parturition, but it is also administered to women who undergo any form of instrumentation (i.e., chorionic villus sampling, amniocentesis, etc.), or who experience spontaneous abortion, or procedural abortion, or abruptio placentae.
The USMLE wants you to know that you do indirect Coomb’s test at the first trimester checkup. This will take the mother’s serum, mix it with laboratory Rh (+) RBCS, and see if there’s agglutination. If there’s agglutination, this says the mother has developed antibodies against Rh antigen from the prior pregnancy and will NOT go on to receive RhoGAM later in pregnancy. In other words, if she’s already made antibodies against Rh antigen, it’s too late to give her RhoGAM because it won’t change anything.
ABO type:
USMLE will usually give you an O+ mom in her first pregnancy, and they will say the fetus is either A or B blood type.
Antibodies we normally produce against opposing blood types (e.g., person with B blood has antibodies against A blood) are IgM and do not cross the placenta.
However patients with O blood will have a fraction of their antibodies that are IgG and can cross the placenta.
This means a mother who is O blood, in her first pregnancy, can have a fetus who experiences in utero hemolysis, or a neonate who experiences hemolytic disease of the newborn. The fetus/neonate must be A or B blood type.
A couple high-yield points here:
- ABO type hemolytic disease of the newborn can absolutely occur in a pregnancy where the mom is Rh negative. But the USMLE will always give you an Rh + mom because they want to assess that you even know what ABO type HDN is. In other words, the question writers are aware that some students might see Rh negative status, and without even understanding the mechanisms, guess the diagnosis of HDN correctly. So they want to weed out students who haven’t heard of ABO type HDN by giving you an Rh + mom.
- ABO type hemolytic disease of the newborn can occur in any numbered pregnancy. The USMLE will usually give you a first pregnancy because the mother with O blood who has IgG antibodies had these antibodies prior to the first pregnancy.
Bottom line:
Rh (-) HDN = 2nd pregnancy onward in Rh – mom with Rh + fetus.
ABO HDN = 1st pregnancy in O mom with A or B fetus. Mom can be Rh – and in 2nd pregnancy onward, but USMLE will give you 1st pregnancy with an O+ mom.
