Prenatal Screening Tests
Prenatal screening tests can identify women at high risk for having a baby with certain types of genetic disorders or birth defects. If a screening test indicates high risk for an abnormality, women have the option of undergoing diagnostic testing (CVS or amniocentesis) for a more definite diagnosis.
Several screening tests are available.
Sequential Integrated Screening
Sequential integrated screening is offered to all pregnant women by the state of California. This non-invasive screening is performed in two steps.
In the first step, which is performed between 10 and 14 weeks of pregnancy, a blood sample is taken from the mother and a nuchal translucency ultrasound is performed to measure the amount of fluid at the back of the baby's neck. If the blood test is scheduled prior to the ultrasound, we can provide the results at the end of the ultrasound appointment. The results of the blood test, the nuchal translucency measurement and the mother's age are used to estimate the risk for Down syndrome and trisomy 18.
The second step is a maternal blood test between 15 to 20 weeks of pregnancy. When the results of this blood test are combined with the results from the first trimester blood test and nuchal translucency ultrasound, the detection rate for Down syndrome increases. This test also provides a personal risk assessment for having a fetus with trisomy 18, Smith-Lemli-Opitz syndrome, an open neural tube defect or an abdominal wall defect. For later stages of pregnancy, modified screening tests are available.
Cell-Free DNA Screening
Cell-free DNA screening is a non-invasive test that screens for chromosome abnormalities by analyzing fragments of fetal DNA in the mother's bloodstream. It can be performed after 10 weeks of pregnancy. Currently, it is offered to women at increased risk of having a baby with chromosome abnormalities.
This method screens for trisomy 13, trisomy 18, trisomy 21 (more commonly known as Down syndrome) or a sex chromosome abnormality. The test's accuracy varies for each of these conditions.
UCSF Health medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.
Prenatal Diagnostic Center
1825 Fourth St., Third Floor
San Francisco, CA 94158
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FAQ: Prenatal Tests
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