## **Core Concept**
Feto-maternal transfusion refers to the leakage of fetal blood into the maternal circulation, which can lead to isoimmunization and other complications. Detection of feto-maternal transfusion is crucial in managing pregnancies, especially in cases of Rh incompatibility. The test used for detection involves identifying fetal hemoglobin or red blood cells in the maternal circulation.

## **Why the Correct Answer is Right**
The correct answer, **Kleihauer test**, is a method used to detect fetal-maternal hemorrhage. It works by identifying fetal hemoglobin (HbF) in maternal red blood cells. The test involves acid elution of adult hemoglobin from red blood cells on a slide, followed by staining. Cells containing HbF resist elution and appear as dark-staining cells under a microscope. This test is specifically useful for detecting and quantifying fetal-maternal transfusion, especially in Rh-negative mothers who may require Rh immunoglobulin prophylaxis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While ultrasound is a critical tool in monitoring fetal development and detecting various pregnancy complications, it is not directly used for detecting feto-maternal transfusion.
- **Option B:** The Coombs test, or antiglobulin test, detects antibodies or complement proteins that are bound to the surface of red blood cells and is used in diagnosing hemolytic disease of the newborn. However, it does not directly detect feto-maternal transfusion.
- **Option D:** The Direct Antiglobulin Test (DAT) is used to detect antibodies or complement proteins that are bound to the surface of red blood cells. While it can be relevant in the context of hemolytic disease of the newborn, it is not the primary method for detecting feto-maternal transfusion.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that the Kleihauer test is particularly important in Rh-negative pregnant women. If a significant feto-maternal transfusion occurs, these women may develop antibodies against Rh-positive blood, which can complicate future pregnancies. The test helps in determining the dose of Rh immunoglobulin needed to prevent isoimmunization.

## **Correct Answer: C. Kleihauer test**