What is Trisomy 18?

Trisomy 18 syndrome is a rare chromosomal disorder in which all or a portion of chromosome 18 appears three times (trisomy), rather than twice in each cell of the body. This extra copy changes how the baby’s body and brain develop, which can cause unusual physical features, structural birth defects and developmental disabilities. Chromosome abnormalities are usually present at conception, the result of errors in egg or sperm formation. In rare cases chromosomes abnormalities are inherited. The majority of babies born with trisomy 18 are female. Trisomy 18 can be diagnosed prenatally by amniocentesis and might be found during prenatal screening or by chromosome testing after the baby is born.

One factor that increases the risk for having a baby with trisomy 18 is the mother’s age. Women who are 35 years or older when they become pregnant are more likely to have a pregnancy affected by trisomy 18 than women who become pregnant at a younger age.

Rates of Trisomy 18: 2018–2020​

Overall​

By Age

​By Race and Ethnicity

*Per 10,000 live births

**No cases in 2018–​2020

Data Source: California Birth Defects Monitoring Program Registry, 2018–​2020, California 10-County Catchment Area (Fresno, Kern, Kings, Madera, Merced, Orange, San Diego, San Joaquin, Stanislaus, and Tulare)

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