Typically, an individual has 23 pairs of chromosomes with 46 chromosomes all in all. The first 22 pairs of chromosomes are numbered according to size, and the last pair are known as the sex chromosomes as they typically determine gender. A baby inherits a set of 23 chromosomes from the mother and a set of 23 chromosomes from the father.
More information about chromosomes can be found at the National Institute of Health Genetics Home Reference.
A trisomy chromosome condition occurs when a person receives extra chromosome material on one of those chromosomes. The most common trisomy chromosome conditions include Down syndrome (also known as Trisomy 21), Trisomy 18, and Trisomy 13.
The chance for a pregnancy to be affected by one of these conditions increases as the age of the mother increases as shown by this chart of maternal-age prevalence, but everyone has some chance to have a baby with a trisomy condition. It is rare for these conditions to run in a family (be inherited), and there is nothing that can be done to cause or prevent a trisomy condition.
Down syndrome or Trisomy 21
Down syndrome is caused by an extra or partial copy of chromosome 21 and affects about 250,000 people in the US. (1) The overall miscarriage rate for Down syndrome between a diagnosis by CVS and term is approximately 32%, and 25% between amniocentesis and term. (2) Most people with Down syndrome have mild to moderate intellectual disabilities and actively participate in their families, communities, and schools with support. Babies with Down syndrome also have low muscle tone and increased chances for certain health conditions, such as heart defects and digestive issues. However, most health issues can be successfully treated or managed, and the average life expectancy is about 60. (3)
Trisomy 13 (Patau syndrome) and Trisomy 18 (Edward’s syndrome)
Trisomy 18 is caused by an extra copy of chromosome 18 and impacts about 1 in 5,000 live births. (4) About 72% of Trisomy 18 pregnancies result in miscarriage or stillbirth between 12 weeks to term. (5) Less than 10% of babies born with Trisomy 18 currently survive past the first year (6), but some children and adults can live longer with intensive medical care. Individuals with Trisomy 18 also have significant intellectual disabilities.
Trisomy 13 occurs in about 1 in 16,000 live births and is caused by an extra copy of chromosome 13. (7) About half of Trisomy 13 pregnancies result in miscarriage or stillbirth between 12 weeks to term, (5) and about 5-8% of babies with Trisomy 13 survive past the first year, usually with intensive medical care and significant intellectual disability. (6)
While those who survive with Trisomy 13 and 18 have profound disabilities, they can accomplish delayed milestones and build meaningful relationships with friends and family. Families affected by Trisomy 13 and 18 often say that they want the opportunity to determine how little or how much medical care their fetus or child receives, and they want access to support services and grief counseling as needed. (8)
- Presson AP, Partyka G, Jensen KM, Devine OJ, Rasmussen SA, McCabe LL, McCabe ERB. Current estimate of Down syndrome population prevalence in the United States. J Pediatr. 2013.
- Savva, G. M., Morris, J. K., Mutton, D. E., & Alberman, E. (2006). Maternal age-specific fetal loss rates in Down syndrome pregnancies. Prenatal Diagnosis, 26(6), 499–504.
- Glasson, E.J., Sullivan, S.G., Petterson, B.A., Montogomery, P.D., & Bittles, A.H. (2002). The changing survival profile of people with Down syndrome: implications for genetic counselling. Clinical Genetics, 62: 390-393.
- Morris JK, Savva GM. The risk of fetal loss following a prenatal diagnosis of trisomy 13 or trisomy 18. Am J Med Genet A. 2008 Apr 1;146(7):827-32. doi: 10.1002/ajmg.a.32220.
- Rasmussen et al. Population-based analyses of mortality in trisomy 13 and trisomy 18. Pediatrics. 2003 Apr;111(4 Pt 1):777-84.
For more information about prenatal testing and prenatally diagnosed conditions, click below:
Understanding Chromosome Conditions
Trisomy Conditions (Down syndrome, Trisomy 18, and Trisomy 13)
Sex Chromosome Conditions (Turner syndrome, Klinefelter syndrome)
Neural Tube Defects (Spina Bifida)
Microdeletions (Angelman/Prader-Willi, 22q, Wolf-Hirschhorn, Cri-du-Chat, Jacobsen, and Williams syndrome)
By using this online tool, you agree to the following:
1. You will not repost the digital file(s) on any other website—even if it’s for a good cause—and you will not email the digital file to anyone other than yourself.
2. You will not print the digital file except for your own personal use.
3. You will respect our copyright and not reuse the text or images or allow/enable someone else to reuse them. Reusing our content or images for any purpose is a violation of federal law.