WinRho Dosing for Primigravid Patient at 36 Weeks Gestation
For routine antenatal prophylaxis at 36 weeks gestation in a 74 kg primigravid woman, administer 300 mcg (1500 IU) of RhIg intramuscularly, regardless of maternal weight. 1
Standard Antenatal Prophylaxis Protocol
The standard dose for antenatal prophylaxis is 300 mcg (1500 IU) administered at approximately 28 weeks gestation, with a repeat dose of 300 mcg (1500 IU) given within 72 hours after delivery if the infant is RhD-positive. 1
At 36 weeks gestation (beyond the standard 28-week timing), this patient should still receive the full 300 mcg dose if she has not yet received antenatal prophylaxis. 2
The maternal weight of 74 kg does not affect the standard dosing—RhIg dosing is not weight-based for routine prophylaxis. 1
Dosing Considerations Based on Timing
If the patient already received her 28-week dose and delivery occurs within 3 weeks of that dose, the postpartum dose may be withheld unless there is evidence of fetomaternal hemorrhage exceeding 15 mL of fetal red blood cells. 1
However, if she has not yet received any antenatal prophylaxis, administering 300 mcg at 36 weeks is appropriate, as the goal is to maintain anti-D levels above the threshold needed to prevent alloimmunization throughout the remainder of pregnancy. 1
Route of Administration
WinRho can be administered either intramuscularly or intravenously, with both routes showing equivalent efficacy in preventing RhD alloimmunization. 3
For IM administration, inject into the deltoid muscle of the upper arm or lateral thigh muscle—avoid the gluteal region due to risk of sciatic nerve injury. 1
IV administration results in immediately high anti-D levels, while IM administration shows similar concentrations by 2-3 weeks post-injection. 3
Assessment for Excess Fetomaternal Hemorrhage
The standard 300 mcg dose provides sufficient antibody coverage for fetomaternal hemorrhage of 15 mL or less of fetal red blood cells (approximately 30 mL of whole fetal blood). 1
Consider quantitative testing for fetomaternal hemorrhage if there has been any placental trauma, abdominal trauma, placental abruption, or significant bleeding, as these events carry substantial risk of hemorrhage exceeding 30 mL. 4
If fetomaternal hemorrhage exceeds 15 mL of fetal red blood cells, administer additional 10 mcg of anti-D for every 0.5 mL of fetal red blood cells beyond the initial 15 mL. 1, 4
Critical Timing Considerations
While the optimal window is within 72 hours of a sensitizing event, anti-D should still be administered even if beyond 72 hours, as some protection is afforded up to 28 days after the event. 1, 4
The half-life of IgG is 23-26 days, so maintaining adequate anti-D levels throughout the third trimester is essential to prevent immune response to RhD-positive fetal cells. 1
Common Pitfalls to Avoid
Do not reduce the dose based on maternal weight—the 300 mcg dose is standard regardless of body mass. 1
Do not withhold the postpartum dose simply because antenatal prophylaxis was given; a repeat dose is required within 72 hours of delivery if the infant is RhD-positive, unless delivery occurs within 3 weeks of the last dose AND there is no excess fetomaternal hemorrhage. 1
Do not inject intravenously if using an IM preparation, and never inject the neonate. 1