Maternal D alloimmunization (prevention): Clinical sciences

Maternal D alloimmunization, sometimes called isoimmunization, occurs when a pregnant patient with Rh-negative blood type is exposed to Rh-positive blood and develops antibodies against the Rh-factor antigen. Maternal sensitization can occur when the fetus is Rh-positive, either during pregnancy, or during delivery, which is when fetal-maternal hemorrhage is most likely to occur. Then, in subsequent pregnancies with an Rh-positive fetus, maternal anti-D antibodies can cross the placenta and attack the fetus’ red blood cells, causing fetal anemia. Severe cases can result in fetal hydrops and death.

Your first step when evaluating a pregnant patient who is Rh negative is to obtain a focused history. The initial prenatal visit is an ideal opportunity to evaluate a patient’s risk and provide education about sensitization. Additionally, the 28-week visit is the recommended time to administer anti-D immune globulin for routine prophylaxis. Keep in mind that maternal D alloimmunization can occur in asymptomatic patients who don’t receive immune globulin prophylaxis at 28 weeks, because some antenatal mixing of fetal and maternal blood occurs despite the separate circulations.

Alright, other important findings include vaginal bleeding, or other sensitizing events in pregnancy that could have caused maternal exposure to fetal red blood cells, such as abdominal trauma, chorionic villus sampling, amniocentesis, cordocentesis, and external cephalic version. Additionally, spontaneous abortion, ectopic pregnancy, and pregnancy-related uterine curettage can also cause maternal exposure.

Ask patients who are known to be Rh-negative if they have received immune globulin in the past 12 weeks, because this could result in a positive antibody screen. Knowing the paternal blood type can also be helpful because Rh-negative patients are not at risk of Rh D alloimmunization when paternity is certain and paternal blood type is Rh-negative. Finally, gestational age is important because it influences the minimum dose of anti-D immune globulin required when antepartum bleeding or another sensitizing event occurs.

Your next step is to obtain an ABO and RhD blood type and antibody screen. This is a routine blood test for all patients at the initial prenatal visit, and upon admission for labor and delivery. For Rh-negative patients, the antibody screen is also repeated at 28 weeks of pregnancy. However, if a patient is at risk for sensitization at other times during gestation, repeat the antibody screen if the patient is Rh-negative, or if their blood type is unknown. Of note, this should be done before anti-D globulin is administered, otherwise that could cause false-positive results.

Most patients who are Rh D negative will have a negative anti-D antibody screen. Your next step for these patients is to assess for vaginal bleeding or another sensitizing event. If a sensitizing event has occurred, manage the patient according to obstetrical guidelines, and administer anti-D globulin. Ok, now, delivery is a high-risk event for Rh D alloimmunization in Rh negative individuals. Within the first 72 hours postpartum, determine the Rh D status of the infant. If the infant is Rh-negative, no further treatment is necessary. If the infant is Rh-positive, your next step is to assess for fetal-maternal hemorrhage. Screening for fetal-maternal hemorrhage begins with the erythrocyte rosette screen, which can detect a volume of fetal blood greater than 2 milliliters in the maternal circulation. If the rosette screen is negative, administer the routine 300-microgram dose of immune globulin. However, if the screen is positive, quantify the amount of fetal-maternal hemorrhage with either a Kleihauer-Betke test or flow cytometry. If a hemorrhage larger than 30 milliliters of fetal whole blood, or 15 milliliters of fetal red blood cells has occurred, additional vials of anti-D immune globulin should be given.

Here’s a high-yield fact! A 300-microgram dose of anti-D immune globulin is sufficient for a hemorrhage of up to 30 milliliters of fetal whole blood or 15 milliliters of fetal red blood cells.

The next group of potentially sensitizing events includes fetal demise or maternal abdominal trauma in the second or third trimesters and persistent antenatal bleeding. If any of these events occur, determine the volume of fetal-maternal hemorrhage with a Kleihauer-Betke test or flow cytometry, and administer the appropriate amount of anti-D immune globulin. If pregnancy continues, re-evaluate the patient at 28 weeks and after delivery for routine prophylaxis as indicated.