What is a Chromosomal Abnormality?


By Lisa Pecos

Chromosomes are structures that carry our genetic material, and more specifically, genes. They are located within every cell of our body, within a structure called the nucleus. When there is an abnormality in the structure or number of one or more chromosomes, this is called a chromosomal anomaly, or abnormality. These abnormalities can occur either at the time of fertilization or during pregnancy and cause negative effects on the growth and development of the baby.

A chromosomal abnormality differs from a single-gene genetic disorder in that an individual can have the normal number of chromosomes, 46, that are structurally sound and still have a genetic disorder. This is because there is a mutation of only one gene, located on a chromosome, that does not affect the structure or number of chromosomes.

What Are The Types of Chromosomal Abnormalities?

Structural changes in a chromosome can include:

  • Deletion – occurs when a section of a chromosome is missing
  • Duplication – occurs when there is extra genetic material on a chromosome because a segment was duplicated and added to the same chromosome
  • Translocation – occurs when a portion of one chromosome detaches and reattaches to a different chromosome
  • Inversion – occurs when a portion of one chromosome detaches, is flipped upside down, and reattaches to the same chromosome
  • Rings – occurs when a portion of a chromosome has broken off forming a ring.

How Does a Chromosomal Abnormality Occur?

Chromosomal abnormalities typically occur as an error in cell division. There are two cell division processes in the body:

  1. Meiosis – cell division of developing egg and sperm cells that will then join together (an egg cell with a sperm cell) during fertilization.
  2. Mitosis – cell division of all cells that are not egg and sperm cells. Mitosis begins after fertilization and occurs throughout our entire lives to produce skin cells, blood cells, and any other cells in our bodies that need to be replaced due to damage or death.

Chromosomal abnormalities can also occur due to factors such as advanced maternal age at the time of pregnancy.

How Do I Know if My Baby Is At Risk of Having a Chromosomal Abnormality?

If you are at risk of having a child with a chromosomal abnormality, your doctor may recommend prenatal testing. Prenatal genetic testing is a genetic screening tool that can be performed as early as week 10 in your pregnancy using nothing more than a blood draw from the mother. You may be at risk of having a child with a chromosomal abnormality if you:

  • Are of advanced maternal age 35 or older
  • Have previously given birth to a child with a chromosomal abnormality
  • Have a family history of a certain genetic disorder
  • Have additional risk factors dictated to you by your doctor

Results from the prenatal screening will indicate to your doctor the risk that your child might be born with a chromosomal abnormality. If you fall under one of the categories listed above, speak to your doctor or with a genetic counselor to better understand your risks as well as your testing options.


  • http://www.stanfordchildrens.org/en/topic/default?id=how-chromosome-abnormalities-happen-meiosis-mitosis-maternal-age-environment-90-P02126
  • https://www.genome.gov/11508982/chromosome-abnormalities-fact-sheet/
  • http://www.nature.com/scitable/definition/spindle-fibers-304
  • http://d-scholarship.pitt.edu/6743/
  • http://www2.le.ac.uk/departments/genetics/vgec/healthprof/topics/patterns-of-inheritance/chromosomal-abnormalities
  • http://www.marchofdimes.org/baby/chromosomal-conditions.aspx