Chromosome abnormality (trisomy and monosomy)

A chromosome abnormality is a change in the usual number or structure of chromosomes. Numerical abnormalities include trisomy (an extra copy of a chromosome, such as trisomy 21 in Down syndrome) and monosomy (a missing chromosome). These are common causes of miscarriage and are a key reason embryos may be classified as aneuploid on PGT-A. Structural chromosome abnormalities involve a rearrangement of chromosome material, such as a translocation, deletion, duplication or inversion, and can sometimes run in families even when a parent is an unaffected carrier. A karyotype test can identify structural abnormalities in a person's own chromosomes, while PGT-A or PGT-SR can assess embryos for numerical or structural chromosome problems relevant to that couple. A finding of chromosome abnormality is a medical description, not a reflection of anyone's worth, and genetic counselling can help explain what a specific result means for future family planning.

Also known as: trisomy, monosomy, aneuploidy in pregnancy

Last reviewed: 9 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.