Prenatal Genetics FAQ

What are Down syndrome, Trisomy 18 and Trisomy 13?

Down syndrome (also known as trisomy 21), trisomy 18 and trisomy 13 are chromosomal abnormalities involving an extra copy of chromosome #21, chromosome #18 and chromosome #13, respectively. These chromosome abnormalities occur after a chance mistake in cell division. Although such abnormalities can occur in any pregnancy, they happen more often with increasing maternal age. Down syndrome causes characteristic facial appearance, intellectual disability and a greater tendency for other birth defects and health-related issues. Trisomy 18 and trisomy 13 are more severe abnormalities, usually resulting in multiple birth defects and neonatal death.

How is First Trimester Screening performed?

Genetic counseling and ordering of the screening studies can be coordinated through the Prenatal Diagnostic Center of The Harvey Institute. The staff is certified to perform the nuchal translucency and nasal bone ultrasound at GBMC. A finger-stick blood sample for the free B-hCG and PAPP-A assays is performed in conjunction with the ultrasound exam and genetic counseling.

What is Genetic Counseling?

Genetic counseling is an educational process during which the known or potential risks of the pregnancy, capabilities and limitations of the first trimester screening, and other testing options are discussed. Genetic counseling involves constructing detailed family, medical and pregnancy histories to assess possible risk factors.

How accurate is the First Trimester screening?

Studies have shown that more than 90% of pregnancies with Down syndrome, trisomy 18 and trisomy 13 will have a positive test result. Up to 10% of Down syndrome, trisomy 18 and trisomy 13 cases will not be detected. Fetuses with other chromosome disorders or birth defects may not be detected by first trimester screenings.

What do positive and negative results mean?

A positive test result indicates an increased risk for Down syndrome, trisomy 18 or trisomy 13 above the accepted baseline cut-off values. The Down syndrome risk cut-off used is 1/311. A positive results for trisomy 18 or trisomy 13 is indicated by a risk of 1/150 (0.67%) or greater. A positive test result is not diagnostic. It indicated that an increased risk for a problem exists and that additional testing should be considered. Similarly, a negative result does now rule out these abnormalities, but instead decreases the specific risks for Down Syndrom, trisomy 18 and trisomy 13 in pregnancy.

Is there a need for additional testing?

If a positive test result is obtained, the genetic counselor will explain the results and review options for further testing, which may include CVS or amniocentesis and high-resolution ultrasounds. If the results of the screening are within the normal range, maternal serum AFP screening for neural tube defects is recommended at 16 weeks. A detailed ultrasound to screen for neural tube defects and other birth defects is recommended at 18-20 weeks of pregnancy. Women whose results are in the normal range whose risk is only moderately decreased will be offered sequential screening.

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