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Chromosomal mosaicism in human embryos occurs when cells within the same embryo have different chromosomal compositions, resulting from errors in mitotic cell division after fertilization. Some cells may have a normal chromosomal number (euploid), while others may have abnormalities, such as extra or missing chromosomes (aneuploid). This condition arises during early embryonic development and can vary in severity depending on the proportion, type, and location of affected cells. Chromosomal mosaicism can influence embryo viability, implantation success, and developmental outcomes. It is a key focus in preimplantation genetic testing, as embryos with mosaicism may still result in healthy pregnancies, depending on the nature and extent of the chromosomal abnormalities.