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Prenatal Diagnosis and Treatment Center
First Trimester Screening for Trisomy 21 (Down Syndrome) and Trisomy 18:
Maternal Serum Screening & Nuchal Translucency Sonogram
PATIENT INFORMATION:
FREQUENTLY ASKED QUESTIONS:
What are Down syndrome, trisomy 13, and trisomy 18?
Down syndrome and trisomies 13 & 18 are chromosomal disorders that cause mental retardation and birth defects. Babies with Down syndrome have an extra chromosome #21 (trisomy 21) which causes mental retardation and various medical problems involving the heart, digestive tract, and/or other organ systems. Trisomy 18 (having an extra chromosome #18) and trisomy 13 (having an extra chromosome #13) are more severe disorders which causes profound mental retardation and severe birth defects in many organ systems. Few babies with trisomies 13 or 18 survive more than a few months. Anyone can have a baby with these chromosome abnormalities, however, the chance increases with the mother’s age
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What can first trimester screening tell me about my pregnancy?
First trimester screening is not a diagnostic test, which means it cannot tell you whether your baby has Down syndrome, trisomy 13, or trisomy 18. Instead, the screening provides a probability that the baby might have Down syndrome, trisomy 13, or trisomy 18. This probability, or chance, is based on three criteria: your age, information obtained on a sonogram (ultrasound), and bloodwork. The screening results can either alert you and your doctor that your baby is at an increased risk for one of these chromosome disorders or be reassuring that your baby is at a lower risk for these conditions.
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How is First Trimester Screening performed?
This screening includes a sonogram and maternal blood work performed between 11 1/7 - 13 6/7 weeks of pregnancy. The sonogram will confirm how far along your pregnancy is. In addition, a measurement of the fluid underneath the skin along the back of the baby's neck, called the nuchal translucency (NT), will be taken. A maternal blood sample is used to analyze two chemicals called free beta-human chorionic gonadotropin (hCG) and pregnancy associated plasma protein-A (PAPP-A), which are found in the blood of all pregnant women. In some pregnancies when the baby has Down syndrome, trisomy 13, or trisomy 18, there is extra fluid behind the baby's neck and/or the hCG and PAPP-A results are higher or lower than average. Combining your age-related risk with the NT measurement and blood work provides you with one risk figure for Down syndrome and one risk figure for trisomy 13 or trisomy 18. Your obstetrician will receive your screening results from NTD Laboratories within approximately one week.
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How accurate is First Trimester Screening?
Because this is a screening test, a positive result (showing an increased risk) does not mean that your baby has a problem, only that further diagnostic tests are options for you to consider. Also, a negative or normal result (one that shows a decreased risk) does not mean that the baby will not have a chromosome abnormality. The first trimester screen’s detection rate is about 78% for pregnancies in which the baby has Down syndrome, and is somewhat higher for pregnancies with trisomy 13 or trisomy 18. A nuchal translucency sonogram can be performed without measuring hCG and PAPP-A; however, the detection rate is reduced to about 70%. Finally, this screen is not designed to provide information about the possibility of other chromosome conditions, nor about many other genetic syndromes, genetic disorders, birth defects, or causes of mental retardation.
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Should I still have the second trimester screening (triple/quad screen)?
The second trimester maternal serum screening test, also known as the "triple screen" or "quad screen", is performed between 16-20 weeks. Both of these screens measure chemicals in the mother's blood. Like the first trimester screening, results from a second trimester "triple screen" or "quad screen" can be used to statistically adjust a women's age-related risk for Down syndrome and trisomy 18 (not trisomy 13). In addition, the AFP portion of the screen can identify pregnancies at an increased risk for open neural tube defects such as spina bifida, which first trimester screening does not include. While these screens are certainly an additional screening option after having first trimester screening, it is unclear how to interpret results of the second test in light of the first. Currently most laboratories do not combine results from first trimester and second trimester screening into one overall adjusted risk figure.
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What is the cost of First Trimester Screening?
The screening involves a charge for the ultrasound examination and the blood test. These charges may not be covered by your insurance and you may want to check with your insurance company to see if your specific plan will cover these services. The blood work will be performed by NTD Laboratories at a cost of approximately $160. There is an additional charge for the sonogram which varies over time. As with any service, if your insurance company requires pre-authorization, you are responsible for obtaining it from your OB or primary care physician prior to the appointment.
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How will I receive my First Trimester Screening results?
Your obstetrician will receive the results from NTD Laboratories and will contact you with your results.
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What if the screening shows an increased risk for one of the conditions?
If the screening results indicate that your baby is at an increased risk for either Down syndrome or trisomy 13 or 18, this does NOT mean that your baby necessarily has one of these conditions. A genetic counselor is available to go over your result and to discuss additional testing options such as chorionic villus sampling (CVS) and amniocentesis. CVS and amniocentesis are diagnostic tests that can tell you with greater than 99% accuracy whether or not a baby has a chromosome abnormality. Also, extra fluid behind the baby’s neck (a larger than expected nuchal translucency) is known to be associated with other birth defects like congenital heart defects and skeletal problems.
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How do I schedule an appointment for First Trimester Screening?
Your obstetrician's office can give you more information about scheduling this screen at Johns Hopkins Hospital. You will need a screening card, with information filled in that pertains specifically to you, and a consent form. You will be responsible for bringing your own completed screening card and signed consent form to the appointment with you. Your doctor will provide you with these materials, and you cannot be screened without bringing them to your scheduled appointment. As with any first trimester sonogram, you will need to have a full bladder.
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