Rh alloimmunization prevention
USMLE Step 2 CKDifficulty: easyLast updated:
A 29-year-old woman, gravida 2 para 1, comes for a routine prenatal visit at 28 weeks' gestation. Her pregnancy has been uncomplicated, with no vaginal bleeding, trauma, or invasive procedures. Her blood type is O, Rh(D)-negative, and her antibody screen at this visit is negative. The father of the baby is Rh(D)-positive.
Which of the following is the most appropriate next step?
Answer and explanation
Correct answer: C. Administer Rho(D) immune globulin 300 mcg intramuscularly now
An unsensitized Rh(D)-negative woman carrying a fetus that may be Rh-positive should receive routine antenatal prophylaxis. The source states that Rho(D) immune globulin is given routinely to all Rh-negative pregnant women with no known prior sensitization and a fetus that might be Rh-positive. In the United States, a single 300-mcg dose is given at 28 weeks. Another dose is given within 72 hours after delivery if the newborn is Rh-positive or neonatal typing is not done. Doses are also given after events that may cause fetomaternal hemorrhage, such as vaginal bleeding, pregnancy loss, amniocentesis, or chorionic villus sampling. Routine antenatal dosing matters because sensitization can occur during pregnancy without a recognized event, so waiting for delivery or for bleeding is inadequate.
Each option explained
- A. Repeat the antibody titer every 4 weeks without giving immune globulinIncorrect. Serial titers are used once a woman is already sensitized; she is unsensitized and needs prophylaxis.
- B. Administer Rho(D) immune globulin only if vaginal bleeding occursIncorrect. Prophylaxis is routine at 28 weeks, not only after bleeding.
- C. Administer Rho(D) immune globulin 300 mcg intramuscularly nowCorrect. Unsensitized Rh-negative women with a possibly Rh-positive fetus receive routine Rho(D) immune globulin at 28 weeks (a single 300-mcg dose in the US).
- D. Administer Rho(D) immune globulin only after delivery if the newborn is Rh-positiveIncorrect. A postpartum dose is also given, but omitting the 28-week dose leaves her exposed to antepartum sensitization.
- E. Perform amniocentesis to determine fetal Rh status before any treatmentIncorrect. Amniocentesis is invasive and can itself cause fetomaternal hemorrhage; it is not needed before routine prophylaxis.
Key point
Give Rho(D) immune globulin at 28 weeks to unsensitized Rh-negative women with a possibly Rh-positive fetus, and again within 72 hours of delivery if the newborn is Rh-positive.
From the source
“Rho(D) immune globulin is given routinely to all Rh-negative pregnant women with no known prior sensitization and a fetus that might be Rh-positive.”