**Core Concept**
Fetal-maternal hemorrhage (FMH) refers to the transfer of fetal blood cells into the maternal circulation, which can occur during pregnancy, labor, or postpartum. Accurate estimation of FMH is crucial for diagnosing and managing conditions such as Rh alloimmunization. The test used to estimate FMH involves detecting the presence of fetal red blood cells in the maternal circulation.

**Why the Correct Answer is Right**
The correct answer is the **Kleihauer-Betke test**. This test is a sensitive method for detecting fetal red blood cells in the maternal circulation. It involves treating a maternal blood sample with acid elution, which removes adult hemoglobin from red blood cells, leaving fetal hemoglobin intact. The sample is then stained with a supravital stain, and the number of fetal red blood cells is quantified. This test is used to estimate the amount of FMH and guide management decisions in cases of suspected Rh alloimmunization.

**Why Each Wrong Option is Incorrect**
**Option A:** The **Duffy test** is used to detect antibodies against the Duffy blood group antigen, which is not relevant to estimating FMH.
**Option B:** The **direct Coombs test** is used to detect antibodies or complement on the surface of red blood cells, but it is not specific for detecting fetal red blood cells.
**Option C:** The **indirect Coombs test** is used to detect antibodies against red blood cells in the serum, but it is not used to estimate FMH.

**Clinical Pearl / High-Yield Fact**
The Kleihauer-Betke test is a crucial tool in diagnosing and managing conditions related to fetal-maternal hemorrhage. It is essential to remember that a positive result indicates the presence of fetal red blood cells in the maternal circulation, which can lead to Rh alloimmunization and other complications.

**Correct Answer:** C. Kleihauer-Betke test