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Most tests are done to see if you have a particular disease or condition. These are called diagnostic tests. Screening tests show if a person’s chance is high or low for a specific disease or condition. If the chance is increased, then a person can then choose to have diagnostic testing.
First trimester screening identifies pregnancies that are at increased risk for Down syndrome or trisomy 18. Down syndrome is caused when a baby gets an extra copy of chromosome 21. Most individuals with Down syndrome are affected with mild-moderate mental retardation; some individuals with Down syndrome also have birth defects, such as heart defects, and other health problems that can vary from mild to serious. Trisomy 18 is caused when a baby gets an extra copy of chromosome 18. This condition causes profound mental retardation and much more serious birth defects and health problems. Your doctor or genetic counselor can answer questions that you have about these conditions.
First trimester screening is performed between 11-14 weeks of pregnancy. The screen consists of two parts: an ultrasound to measure the amount of fluid at the back of the baby’s neck (called the nuchal translucency) and a maternal blood test to measure two different substances (pregnancy associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG)). The nuchal translucency measurement is often increased in pregnancies affected with chromosome problems like Down syndrome and trisomy 18. Likewise, the levels of PAPP-A and hCG are often different in pregnancies with these chromosome abnormalities. The ultrasound and bloodwork results are combined to provide an estimated risk for both Down syndrome and trisomy 18.
Current data suggests that first trimester screening detects about 85% of pregnancies affected with Down syndrome or trisomy 18. A normal (low risk) result does not guarantee that a baby does not have Down syndrome or trisomy 18. An abnormal (high risk) result only indicates an increased risk – it does not mean that Down syndrome or trisomy 18 is definitely present.
Women who are between 11 and 14 weeks of pregnancy are eligible for first trimester screening. Talk to your doctor or a genetic counselor about the screening. He or she can explain this test in detail and answer your questions. Genetic counselors from the UPMC Center for Medical Genetics are available for consultations with referral from your doctor.
An appointment is made at Magee-Womens Hospital of UPMC or UPMC Mercy between 11 and 14 weeks of pregnancy. Appointments are coordinated through the UPMC Center for Medical Genetics at 412-641-4168 or 1-800-454-8155. At the screening, a small sample of blood is taken from the mother, and an ultrasound is performed.
Multiple marker screening is performed between 15-20 weeks to screen for Down syndrome, trisomy 18, and open neural tube defects (spina bifida and anencephaly. If first trimester screening is performed, then a multiple marker screening for Down syndrome and trisomy 18 is not necessary. However, your doctor might recommend that you still have just the part of multiple marker screening (AFP) that screens for open neural tube defects.
For more information on first trimester screening, or if you have questions about the information provided on this page, please call the UPMC Center for Medical Genetics at 412-641-4168 or 1-800-454-8155.
Revised September 2011