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What is the latest gestational period to measure the fetomaternal hemorrhage?

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The sources are consistent and clear. Here is a synthesis of what the textbooks say:

Fetomaternal Hemorrhage (FMH) — Gestational Timing for Measurement

The Kleihauer-Betke (KB) test is the standard method used to detect and quantify FMH. The relevant gestational threshold for performing this test is:
> 12 weeks of gestation
Multiple authoritative sources agree on this threshold:
  • Tintinalli's Emergency Medicine states: "Perform a Kleihauer-Betke analysis on pregnant women who are >12 weeks of gestation."
  • Mulholland and Greenfield's Surgery notes: "Kleihauer-Betke testing is advocated for all Rh-negative mothers with >12 weeks of gestation."
  • Mulholland also includes it as a surgical key point: "A Kleihauer-Betke test should be performed in all pregnant trauma patients >12 weeks of gestation."
The rationale is physiological:
  • At 12 weeks, total fetal blood volume is only ~4.2 mL — too small for a clinically significant FMH to occur before this point.
  • FMH is rare in the first trimester; fetal erythrocytes in maternal circulation are less frequent, with an increase in both frequency and volume in the second and third trimesters.
  • The largest FMHs are noted to occur during labor and delivery.

RhIG Dosing Tied to This Threshold

Gestational AgeRho(D) Immunoglobulin Dose
≤ 12 weeks50 micrograms IM
> 12 weeks (2nd & 3rd trimester)300 micrograms IM
The KB test is used to determine if doses >300 micrograms are needed (e.g., massive FMH >30 mL of fetal blood), and must be given within 72 hours of the event.
Bottom line: FMH measurement with the Kleihauer-Betke test is indicated from >12 weeks of gestation onward, as this is when the fetal blood volume becomes large enough for clinically significant transplacental hemorrhage to occur.
— Tintinalli's Emergency Medicine, Mulholland and Greenfield's Surgery, Creasy & Resnik's Maternal-Fetal Medicine
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