**Core Concept**
The presence of fetal cells in maternal blood is a result of the process of microchimerism, where small numbers of fetal cells can cross the placenta into the maternal circulation. This phenomenon is a consequence of the unique immunological relationship between the mother and the fetus.

**Why the Correct Answer is Right**
The correct answer is based on the principle of cell-free DNA (cfDNA) analysis. cfDNA is a mixture of DNA fragments from both maternal and fetal sources, which can be detected in maternal blood. The fetal DNA fragments are derived from the placenta and are present in much lower concentrations than the maternal DNA. The use of sensitive molecular techniques, such as next-generation sequencing, allows for the detection and analysis of fetal DNA, enabling the non-invasive prenatal testing (NIPT) of various conditions, including chromosomal aneuploidies. The presence of fetal cells in maternal blood is also a result of the process of cell-free DNA transfer, where intact cells are not transferred, but rather, DNA is released into the maternal circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the detection of fetal cells in maternal blood. While it may be related to other aspects of maternal-fetal medicine, it is not the correct answer.

**Option B:** This option is not a valid method for detecting fetal cells in maternal blood. While it may be related to other aspects of maternal-fetal medicine, it is not the correct answer.

**Option C:** This option is not a valid method for detecting fetal cells in maternal blood. While it may be related to other aspects of maternal-fetal medicine, it is not the correct answer.

**Clinical Pearl / High-Yield Fact**
The detection of fetal cells in maternal blood using cell-free DNA analysis has revolutionized the field of prenatal testing, enabling the non-invasive diagnosis of various conditions, including chromosomal aneuploidies. This technology has also led to the development of new strategies for prenatal care, including the detection of fetal sex and the analysis of fetal genetic material.

**Correct Answer:** D. Cell-free DNA analysis.