Chromosomal Mosaicism in the Placenta

The clinical implications of placental chromosomal mosaicism can be challenging for patients and health care providers. Key considerations include the specific characteristics of the chromosomal abnormality (such as size, gene content, and copy number), the timing of the mosaicism's onset during embryogenesis or fetal development, the types of tissues involved, and the level of mosaicism (the ratio of normal to abnormal cells within those tissues). Genetic counseling can help inform patients about the chances of having a live-born child with a chromosomal abnormality. Each case requires individual assessment to provide accurate guidance. This chapter will explore the clinical implications of detecting mosaicism at 3 critical diagnostic stages: (1) chorionic villus sampling (CVS); (2) amniocentesis; and (3) cell-free DNA (cfDNA) testing.