If the father is heterozygous, there is a 50% chance that the fetus will be RhD positive, as he will randomly pass on either the RhD positive allele or not.If a fetus is RhD positive and the mother is RhD negative, the mother is at risk of RhD alloimmunization, where the mother mounts an immune response (develops antibodies) to fetal red blood cells. This usually has minimal effect on the first such pregnancy; but, in a second such pregnancy, pre-existing maternal antibodies to RhD antigens on fetal red blood cells often leads to erythroblastosis fetalis, a condition which can be fatal to the fetus. In countries without Rh immune globulin (RhIG) protocols, as many as 14% of affected fetuses are stillborn and 50% of live births result in neonatal death or brain injury.Because of this severe complication, the American College of Obstetricians and Gynecologists (ACOG) recommends that all RhD negative mothers, regardless of fetal blood type, receive RhIG at about 28 weeks gestation, and again shortly after delivery in the case of an RhD positive or RhD unknown baby. It should be given within 3 days of a potential exposure to Rh positive blood from the baby such as may occur during second or third trimester miscarriage, amniocentesis, cordocentesis, chorionic villus sampling, external cephalic version, trauma, or delivery. It should be considered after first trimester miscarriage. The 28 weeks recommendation comes from the fact that 92% of women who develop an anti-D during pregnancy do so at or after 28 weeks gestation.
It is given by intramuscular injection as part of modern routine antenatal care. Despite excellent results, the medication retains an FDA Pregnancy Category C.RhIG is recommended in the UK after antenatal pathological events that are likely to cause a feto–maternal hemorrhage. Applicable pathologic events include accidents which may induce fetomaternal hemorrhage (motor vehicle accidents, falls, abdominal trauma), following obstetric/gynecologic procedures during pregnancy, and at the time of threatened- or spontaneous-/elective abortions, regardless of gestational age. There is insufficient evidence that the use of Rho(D) immune globulin after a spontaneous miscarriage is needed and a Cochrane review recommends that local practices be followed.In an RhD negative mother, RhIG can prevent temporary sensitization of the maternal immune system to RhD antigens, which can cause rhesus disease in the current or in subsequent pregnancies. With the widespread use of RhIG, Rh disease of the fetus and newborn has almost disappeared in the developed world. The risk that an RhD negative mother can be alloimmunized by a RhD positive fetus can be reduced from approximately 16% to less than 0.1% by the appropriate administration of RhIG.Rh immune globulin is composed of IgG antibodies and therefore is able to cross the placenta. In rare cases this can cause a baby to have a weakly positive direct antiglobulin test (DAT) due to sensitization of fetal cells from mothers who have received multiple doses of RhIG. However, no treatment is necessary as the clinical course is benign.