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Down syndrome
pregnancy termination
prenatal diagnosis


chromosome abnormalities

picture of chromosomes About 1 in 540 pregnancies in California is diagnosed with a chromosome abnormality—extra or missing genetic material, visible under the microscope—causing unusual physical features, structural birth defects and mental retardation.

Trisomy—47 chromosomes instead of the usual 46—is the most common type of chromosome abnormality. Of these, Down syndrome is the most well known.

Chromosome abnormalities are usually present at conception, the result of errors in egg or sperm formation. In rare cases, chromosome abnormalities are inherited.


bullet Trisomies increase with the mother's age. Women over age 35 are routinely offered prenatal diagnosis because of their higher risk.
bullet Racial/ethnic variation in chromosome abnormalities partly reflect differences in mothers' ages; later childbearing results in higher rates. Cultural differences in use of prenatal diagnosis and termination of affected pregnancies also play a role.


bullet Studies can help better understand chromosome abnormalities, for example, identifying associated birth defects or determining their impact in stillbirths or infant death.
bullet The presence of extra/missing chromosome material may intensify the effect of nongenetic risk factors, providing insight into the development of structural birth defects. For example, one California Birth Defects Monitoring Program study looked at Down syndrome and smoking/caffeine consumption during pregnancy.

bullet Down syndrome
bullet Down syndrome: study findings (environmental exposures and risk factors that may promote miscarriage in affected pregnancies)
bullet Down syndrome: associated birth defects
bullet Down syndrome: impact of prenatal diagnosis


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