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California BIRTH Defects Monitoring

What is Trisomy 18?

Trisomy 18 syndrome is a rare chromosomal disorder in which all or a large portion of chromosome 18 is present 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 distinctive 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 chromosome abnormalities are inherited. The majority of babies born with trisomy 18 are female. Trisomy 18 can be diagnosed prenatally by amniocentesis or chorionic villus sampling.

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.

Mother's Age
​Prevalence Rate (per 10,000 live births)
​All Births


Mother's Race/Ethnicity*
​Prevalence Rate (per 10,000 live births)
​Asian/Pacific Islander

*White, Black, Asian/Pacific Islander categories are all non-Hispanic

 Data Source: California Birth Defects Monitoring Program Registry, 2012-2016 (PDF)

California Central Valley Counties (Fresno, Kern, Kings, Madera, Merced, San Joaquin, Stanislaus, and Tulare)

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