Is Rhogam (Rho(D) immune globulin) administration at 34 weeks gestation necessary if it was already given at 28 weeks?

Rhogam Administration at 34 Weeks After 28-Week Dose

Rhogam administration at 34 weeks is not necessary if it was already given at 28 weeks gestation, as long as delivery occurs within 12 weeks of the initial dose. 1

Rationale for Single Antepartum Dose

The FDA-approved labeling for Rhogam clearly states that a single antepartum dose at 26-28 weeks gestation provides adequate protection through delivery, provided that:

• Delivery occurs within 12 weeks of administration (i.e., before 40 weeks)
• There has been no significant fetomaternal hemorrhage in the interim 1

The standard 300 μg (1500 IU) dose of Rhogam administered at 28 weeks provides protection for up to 12 weeks, which typically covers the remainder of pregnancy for most women delivering at term 1.

When Additional Doses May Be Needed

Additional doses of Rhogam during pregnancy are indicated only in specific circumstances:

• If delivery does not occur within 12 weeks after the standard antepartum dose (i.e., after 40 weeks gestation)
• Following significant trauma or procedures that might cause fetomaternal hemorrhage
• When quantitative testing shows fetomaternal hemorrhage exceeding 15 mL of Rh-positive red blood cells 1

Evidence Supporting Single Dose Protocol

Clinical studies have demonstrated that the current protocol of administering Rhogam at 28 weeks gestation and again within 72 hours of delivery has reduced the Rh immunization rate to approximately 0.1-0.2% 1. This represents a 95.9% protection rate compared to no prophylaxis 2.

A Canadian study showed that antenatal prophylaxis with a single injection of Rhogam (300 μg) at 28 weeks was highly effective, with none of the treated women showing evidence of Rh immunization at delivery or at 6-month follow-up 3.

Historical Context

Historically, some protocols called for Rhogam administration at both 28 and 34 weeks. The introduction of antenatal prophylaxis at 28 and/or 34 weeks' gestation reduced the immunization rate by approximately 90% compared to postpartum administration alone 4. However, current evidence and guidelines have established that a single dose at 28 weeks is sufficient for most pregnancies.

Important Considerations

• Postpartum administration is still required if the infant is Rh-positive, regardless of antepartum prophylaxis
• Testing for fetomaternal hemorrhage should be performed at delivery to determine if additional doses are needed
• If delivery occurs within three weeks of the antepartum dose, the postpartum dose may be withheld, but testing for fetomaternal hemorrhage should still be performed 1

Potential Failures of Prophylaxis

Despite appropriate prophylaxis, a small percentage (approximately 0.24-0.31%) of Rh-negative women may still become immunized 2. Failures typically occur due to:

• Small fetal transplacental hemorrhages before the 28-week dose (15% of failures)
• Significant fetal transplacental hemorrhage between 30-38 weeks that exceeds the protection provided by residual passive antibody (18% of failures) 2

While additional doses at 34 weeks might theoretically reduce these failures further, cost-benefit analyses suggest that the current protocol of a single dose at 28 weeks plus postpartum prophylaxis represents the optimal approach for most women 2.