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Rh prevention — USMLE Step 3 MCQ

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HardOB/GYN (Prenatal / Contraception / Screening)Rh preventionUSMLE Step 3

An unsensitized O-negative woman at 30 weeks of gestation has blunt abdominal trauma in a motor-vehicle collision. Fetal monitoring is reassuring, ultrasound shows no abruption, and she received routine Rh(D) immune globulin at 28 weeks. A fetal-cell screen is strongly positive. Which management is most appropriate?

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Correct answer: D — Give Rh(D) immune globulin and quantify hemorrhage for additional dosing

Explanation lettering: E = shown as A · D = shown as B · A = shown as C · B = shown as D · C = shown as E

The best answer is “Give Rh(D) immune globulin and quantify hemorrhage for additional dosing”. Abdominal trauma is a new sensitizing event even after routine 28-week prophylaxis. An unsensitized Rh-negative patient should receive Rh immune globulin, and a positive fetal-cell screen prompts quantitative testing, such as flow cytometry or Kleihauer-Betke testing, to estimate hemorrhage volume and calculate additional vials beyond the standard dose. Reassuring fetal monitoring does not eliminate maternal alloimmunization risk.

Reference: ACOG: The Rh Factor and Pregnancy: https://www.acog.org/womens-health/faqs/the-rh-factor-how-it-can-affect-your-pregnancy