Tests and diagnosisBy Mayo Clinic staff
Screening for Down syndrome is offered as a routine part of prenatal care. A mother's age has traditionally been a factor in the decision to screen for Down syndrome. But now, the American Congress of Obstetricians and Gynecologists recommends offering various screening tests for Down syndrome to all pregnant women, regardless of age. Although screening tests aren't perfect, they can help you make decisions about more-invasive diagnostic tests and the course of the pregnancy. If your baby is diagnosed before birth with Down syndrome, you'll also have more time to prepare for caring for a child with special needs. Your health care provider can help you weigh the pros and cons of these tests.
Screening tests during pregnancy
Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome. In the past, blood tests typically have been offered around the 16th week of pregnancy to screen for Down syndrome, spina bifida and various other chromosomal disorders.
Currently, more women are electing the first trimester combined test, done in two steps during week 11 to 13 of pregnancy. The first trimester combined test includes:
- Ultrasound. The doctor uses ultrasound to measure a specific region on the back of a baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this tissue.
- Blood tests. Results of the ultrasound are paired with blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
If early assessment isn't your first priority, you can have full integrated testing, which is done in two parts during the first two trimesters of your pregnancy. The results of the two parts are combined to estimate the risk that your baby has Down syndrome. This test can achieve the same level of detection as the first trimester combined test but with a lower false-positive rate, meaning that fewer women are incorrectly identified as carrying a baby with Down syndrome.
- First trimester. Part one includes an ultrasound to measure nuchal translucency and a blood test to measure PAPP-A.
- Second trimester. Done at 15 to 20 weeks of pregnancy, the quad screen measures your blood level of four pregnancy-associated substances, alpha fetoprotein, estriol, HCG and inhibin A.
Of all the women who undergo screening tests for Down syndrome, about 5 percent are identified as being at risk. But the overall risk of Down syndrome among pregnant women is far below 5 percent. So the screening tests, by design, are much better at identifying women who may be at risk of having a baby with Down syndrome than women who actually have a baby with Down syndrome.
If a screening test indicates a high risk of Down syndrome, a more invasive test may be used to determine whether your baby actually has Down syndrome (diagnostic test).
Diagnostic tests during pregnancy
If your screening test results are positive or worrisome or you're at high risk of having a baby with Down syndrome, you might consider further testing to confirm the diagnosis. Diagnostic tests that can identify Down syndrome include:
- Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test after 15 weeks of gestation. The test carries a 1 in 200 risk of miscarriage.
- Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the ninth and 14th week of pregnancy, this test carries a 1 in 100 risk of miscarriage.
- Percutaneous umbilical blood sampling (PUBS). Blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors generally perform this test after 18 weeks of gestation. This test carries a greater risk of miscarriage than does amniocentesis or chorionic villus sampling. Generally, this test is only done when results of other tests are unclear.
New prenatal tests under study
Researchers are working on improved ways of detecting genetic problems early on, including:
- Preimplantation genetic diagnosis. One option that may become available for couples undergoing in vitro fertilization is testing of the embryo for genetic abnormalities before it's implanted in the womb.
- Analysis of circulating fetal DNA. Although not widely available, new tests that evaluate fetal DNA circulating in the mother's blood may create another option for prenatal diagnosis of Down syndrome and other chromosomal abnormalities.
Diagnostic tests for newborns
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. If your child displays some or all of the characteristics of Down syndrome, your doctor probably will order a test called a chromosomal karyotype. This test is an analysis of your child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is Down syndrome.
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