**Core Concept:**
Feto-maternal hemorrhage occurs when blood from the mother and fetus mixes, which can lead to complications like hemolytic disease of the newborn and immune complex disease. Tests help estimate the amount of this hemorrhage.

**Why the Correct Answer is Right:**
The correct answer, **C. Schirmer's test**, is used to estimate the amount of feto-maternal hemorrhage. Schirmer's test measures the amount of blood in the maternal circulation by assessing the amount of clotting factors and proteins present in the maternal blood. This test is performed by placing a Schirmer's strip in the lower eyelid, and the length of the strip soaked with blood indicates the severity of the hemorrhage.

**Why Each Wrong Option is Incorrect:**
A. **B. Favors:** This option is incorrect because it focuses on favoring the mother's immune system, which is not relevant to estimating the amount of feto-maternal hemorrhage.

B. **Fetal fibronectin (FFN):** Fetal fibronectin is a protein found in the amniotic fluid, not maternal blood. This test is used to detect premature rupture of membranes but is not used to estimate feto-maternal hemorrhage.

D. **Hematocrit:** Hematocrit measures the percentage of red blood cells in the blood, but it does not estimate the amount of feto-maternal hemorrhage.

**Clinical Pearl:**
Schirmer's test is a valuable tool for assessing the severity of feto-maternal hemorrhage and guiding management strategies for both the mother and fetus. Adequate management of feto-maternal hemorrhage requires a comprehensive understanding of the test and its interpretation.