Skip Navigation LinksTrisomy-21

California BIRTH Defects Monitoring

What is Trisomy 21​?

Down syndrome is a condition in which a person has an extra chromosome. Chromosomes are small “packages” of genes in the body. They determine how a baby’s body forms during pregnancy and how the baby’s body functions as it grows in the womb and after birth. Typically, a baby is born with 46 chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy.’ Down syndrome is also referred to as Trisomy 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for the baby.

Even though people with Down syndrome might act and look similar, each person has different abilities. People with Down syndrome usually have an IQ (a measure of intelligence) in the mildly-to-moderately low range and are slower to speak than other children.

Content provided by the Centers for Disease Control and Prevention (CDC). For information and references about trisomy 21, please visit Facts about Down Syndrome​ | CDC ​.

Rates of Trisomy 21: 2018–2020​


​​Rates of Trisomy 21​​​
​Prevalence Rate*
​All Deliveries

By Age

Mothers Age (Years) ​Prevalence Rate*
​24 and under
​40 and over

​By Race and Ethnicity

Mothers Race and Ethnicity
​Prevalence Rate*
​American Indian/Alaskan Native, Native Hawaiian/Pacific Islander, Other/Non-Hispanic

*Per 10,000 live births

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)

Back to Data Sheets​​​​​

Page Last Updated :