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For pregnant women, to assess the risk of your baby having a chromosome disorder, such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18)
Usually between 11 and 14 weeks of pregnancy
A blood sample drawn from a vein in your arm or a blood spot collected on special paper from a finger stick; a nuchal translucency requires a special ultrasound exam.
You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.
The first trimester screening is a combination of two blood tests and a special ultrasound that are used to assess a pregnant woman's risk of carrying a baby with Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). Performing and evaluating them together, plus considering the woman's age, increases both the sensitivity and specificity of the screening results.
If the results of first trimester screening are cause for concern, diagnostic tests such as amniocentesis or chorionic villus sampling (CVS) may be recommended.
Blood is drawn from a vein in the woman's arm or collected from a finger stick. The nuchal translucency ultrasound may be performed from outside the abdomen (transabdominally) or the probe may be inserted into the vagina (transvaginally).
You may be instructed to have a full bladder when having the nuchal translucency ultrasound performed.
The first trimester screening is a combination of tests for PAPP-A, hCG and nuchal translucency that are used to assess the risk that the fetus a pregnant woman is carrying has a chromosome disorder such as Down syndrome (trisomy 21) or Edwards syndrome (trisomy 18). The first trimester screen is one of the options you may be offered for prenatal Down syndrome screening. Other options include the second trimester maternal serum screen and the cell-free fetal DNA test.
One advantage of the first trimester screen is that it gives expecting women the option of earlier screening. However, the first trimester screen does not assess risk of neural tube defects such as spina bifida. This may be assessed in the second trimester as part of the maternal serum screen (triple or quad screen) or on its own.
The screening approach a woman may choose depends on what technology is available and when she first seeks prenatal care. For more information about how first trimester screening fits in with other screening options, see below.
Your healthcare practitioner may offer you the test between 11 and 14 weeks of pregnancy.
A genetic counselor or clinician who can explain the meaning of the results and offer choices about follow up should interpret the screening results.
A mathematical calculation using the results obtained from the PAPP-A, hCG, and nuchal translucency ultrasound is used to determine a numeric risk of a chromosomal defect in the fetus. This risk is compared with an established cut-off. If the risk is higher than the cut-off value (e.g., probability of 1 in 230 or higher), then it is considered a positive screen and the woman may be at increased risk for having a baby with a chromosomal abnormality.
It is important to remember that positive screening tests are not diagnostic of fetal abnormalities. While they do indicate an increased risk, only a very small number of women with a positive first trimester screen have babies who actually have a chromosome disorder.
In pregnancies where the fetus is carrying a chromosomal defect, such as the extra chromosome material that results in Down syndrome or Edwards syndrome, the levels of PAPP-A tend to be decreased, the levels of hCG are significantly increased, and the space at the fetus's neck is larger than normal.
While the first trimester screen can correctly identify approximately 85% of women carrying a fetus with Down syndrome and up to 75% of those with Edwards syndrome, about 5% to 10% of normal pregnancies will have a false-positive result.
If a screening test is positive, more definitive tests are needed to determine and confirm a diagnosis. These may include a diagnostic test such as chorionic villus sampling (CVS) in the first trimester or amniocentesis in the second trimester. While these two procedures are more accurate than screening tests, they are also invasive and carry a small risk of miscarriage and a rare risk of injury to the fetus.
Screening will not detect all cases of fetal abnormalities.
Test results are very dependent on nuchal translucency techniques and the accurate determination of the gestational age of the fetus. If the gestational age of the fetus has not been accurately determined, the results may be either falsely high or low.
In multiple gestation pregnancies (twins, triplets, etc.), calculation of the risk of Down syndrome or Edwards syndrome can be difficult because the amount of PAPP-A and free beta hCG is increased. The nuchal translucency ultrasound, however, is an assessment that is unique to each fetus and can be done independently with ultrasound. A woman with a multiple gestation pregnancy should consult her healthcare practitioner about her options.
About 1 in 700 babies are born with Down syndrome (trisomy 21) each year in the U.S. The condition causes mild to moderate mental retardation and developmental problems and can be associated with congenital heart defects, respiratory and hearing problems, leukemia, and thyroid disorders. Many of the complications of Down syndrome can be treated and the lifespan of those affected has greatly increased in recent years. The risk of having a child with Down syndrome or other chromosomal abnormality increases with the age of the mother. Although the risk of having an affected baby is significantly greater in those older than 35, the majority of Down syndrome babies (about 80%) are born to those under 35 because this age group has the greatest number of children. For this reason, the American College of Obstetricians and Gynecologists has recommended that all pregnant women be offered a screening test for Down syndrome.
Edwards syndrome (trisomy 18) is a condition in which there are three copies of chromosome 18. As with Down syndrome, the risk of carrying a fetus with Edwards syndrome increases with maternal age. Edwards syndrome is associated with multiple abnormalities and is usually fatal, with live-born infants rarely living beyond one year of age. The frequency of this abnormality is much less than Down syndrome, occurring in only 1 in 6,000 live births. For more information, see the Trisomy 18 Foundation web site.
A newer test called cell-free fetal DNA (cffDNA) only requires a blood sample from the pregnant woman and can be used to screen for certain fetal chromosome disorders, including Down syndrome, Edwards syndrome, and Patau syndrome (trisomy 13). It can be performed as early as the tenth week of pregnancy. Invasive diagnostic tests such as chorionic villus sampling (CVS) and amniocentesis are still needed to confirm the results of cffDNA. Since cffDNA screening is a relatively new test, some insurance companies may not cover it and it may not be available everywhere. For more information, see the article on Cell-Free Fetal DNA Testing.
There are several approaches to screening depending on what technology is available where you are and when you first visit your healthcare practitioner for prenatal care. Options include:
• First trimester screening for chromosome abnormalities using the combined tests for PAPP-A, hCG and nuchal translucency ultrasound (first-trimester Down syndrome screen), followed by a blood test for maternal AFP and/or a fetal ultrasound in the second trimester to check for neural tube defects
• Second trimester maternal serum screening (triple or quad screen) to assess risk for chromosome abnormalities and neural tube defects; this may be the only testing done, especially in the case of women who do not visit their healthcare practitioner until the second trimester.
• Cell-free fetal DNA testing in the first or second trimester, followed by a blood test for maternal AFP and/or a fetal ultrasound in the second trimester to check for neural tube defects
Typically, if first trimester screening for chromosome disorders has been performed, then screening in the second trimester is not performed because the risks for Down syndrome and Edwards syndrome have already been assessed. However, if you and your healthcare practitioner wish to use the results of both the first trimester Down syndrome screen combined tests and the second trimester maternal serum screen to assess the risk of chromosome abnormalities, then one of the following approaches is used:
• Integrated screening – involves performing both first trimester Down syndrome screening and second trimester maternal serum screening and not releasing the results until all testing is completed
• Sequential screening – involves performing a first trimester screen; if the screen indicates increased risk for Down syndrome or Edwards syndrome, a diagnostic procedure such as chorionic villus sampling (CVS) is offered. If the screen does not indicate increased risk, then a quad screen in the second trimester is offered and results from both the first and second trimester screens are used in the final risk assessment.
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