Glossary · Definition · v2026.3
Aneuploidy in IVF
Aneuploidy is an abnormal number of chromosomes in an embryo: either a gain or loss from the expected 46. It is the leading cause of implantation failure, miscarriage, and failed IVF transfers. Aneuploidy rate increases significantly with the age of the egg provider.
How Aneuploidy Affects IVF
During IVF, embryos are cultured to blastocyst stage and may be biopsied for preimplantation genetic testing (PGT-A). PGT-A screens for chromosomal aneuploidy by analyzing cells from the trophectoderm. Embryos classified as aneuploid are generally not transferred, as they carry a very low probability of resulting in a healthy live birth. The proportion of aneuploid embryos increases with maternal age: at age 30, approximately 30% of blastocysts are aneuploid; by 40, this rises to 60-80%; by 43, it may exceed 90%. This is the primary biological mechanism behind age-related fertility decline. It is not a failure of IVF: it is a reflection of oocyte biology that IVF makes visible but cannot correct.
Euploid vs Aneuploid
An euploid embryo has the expected 46 chromosomes (23 pairs). An aneuploid embryo has either too many (trisomy) or too few (monosomy) chromosomes. Monosomies are generally lethal and do not implant. Some trisomies (such as trisomy 21, Down syndrome) are viable. Mosaic embryos: those with a mix of euploid and aneuploid cells, represent a middle category with complex clinical implications. The clinical decision of whether to transfer a mosaic embryo depends on the specific chromosome involved, the proportion of abnormal cells, and the availability of euploid alternatives.
Related reference
Used in
This content defines terminology for educational orientation. It does not constitute medical advice.
IVF Daddies Newsletter
Structural briefings on policy, clinical, and regulatory developments.
Weekly updates on family-building governance.