Why is the RhoGAM shot given at 28 weeks?

What does the evidence say about RhoGAM shot timing during pregnancy?

1. ACOG explains that Rh immune globulin at 28 weeks clears Rh-positive fetal cells before an Rh-negative patient forms antibodies and that a dose is generally repeated after delivery of an Rh-positive baby. 1
2. The FDA RhoGAM package insert describes an antepartum dose at 26 to 28 weeks, a postpartum dose when indicated, and repeat-dose considerations when a dose is given early. 2
3. StatPearls describes Rh immune globulin as anti-D antibody and lists use at weeks 26 to 28 and within 72 hours after an Rh-positive delivery; dosing can vary with the indication and the size of a blood transfer. 3
4. MedlinePlus explains that fetal red cells can cross into an Rh-negative mother's blood and that Rh immune globulin is used during pregnancy to reduce Rh-incompatibility complications. 4
5. The NCBI Blood Groups and Red Cell Antigens chapter explains that an Rh-negative mother can form anti-D antibodies after exposure to an Rh-positive fetus and that those antibodies can cause hemolytic disease. 5

Is RhoGAM shot timing safe in each trimester?

• First trimester. Blood type and antibody screening identify whether Rh immune globulin may be relevant. Whether it is needed after an early bleeding event or pregnancy loss depends on gestational age, event, and current guidance.
• Second trimester. Routine antepartum Rh immune globulin is commonly given at about 28 weeks if the patient is Rh-negative and not already sensitized. The injection does not erase antibodies that are already present.
• Third trimester. If the newborn is Rh-positive or unknown, a postpartum dose is usually given within 72 hours. Bleeding, trauma, procedures, or an attempted version may require additional assessment.