**Core Concept**
Fetomaternal transfusion, also known as fetomaternal hemorrhage (FMH), refers to the transfer of fetal blood cells into the maternal circulation during pregnancy or delivery. This phenomenon has significant implications for the management of RhD-negative women to prevent alloimmunization.

**Why the Correct Answer is Right**
The correct answer is 10% because studies have consistently shown that fetomaternal transfusion of more than 30 ml occurs in approximately 10% of women at delivery. This is due to the mechanical disruption of fetal-maternal interfaces during labor and delivery, such as fetal scalp blood sampling, fetal scalp electrodes, or other invasive procedures. The amount of blood transfused is often correlated with the duration and intensity of labor, as well as the presence of fetal distress or other complications. The detection of fetomaternal transfusion is crucial in RhD-negative women to administer RhD immunoglobulin and prevent alloimmunization, which can lead to hemolytic disease of the newborn.

**Why Each Wrong Option is Incorrect**
**Option A:** Less than 1% is an underestimation, as fetomaternal transfusion is a relatively common occurrence during delivery.
**Option B:** 5% is still lower than the actual incidence, which is closer to 10%.
**Option D:** 15% is an overestimation, as the actual incidence is around 10%.

**Clinical Pearl / High-Yield Fact**
To detect fetomaternal transfusion, the Kleihauer-Betke test can be performed on a maternal blood sample. This test involves treating the blood with acid to remove adult hemoglobin and then staining the remaining fetal hemoglobin, which is more resistant to acid. The test can help quantify the amount of fetomaternal transfusion and guide the administration of RhD immunoglobulin.

✓ Correct Answer: C. 10%