Down Syndrome - Prenatal Testing

Prenatal Testing

In the United States, ACOG guidelines recommend that screening be offered to all women, regardless of their age Many standard prenatal screens can discover Down syndrome. Genetic counseling along with genetic testing, such as amniocentesis, chorionic villus sampling (CVS), or percutaneous umbilical cord blood sampling (PUBS) can be offered to families who may have an increased chance of having a child with Down syndrome, or where normal prenatal exams indicate possible problems.

Noninvasive screening tests asses the risk of Down syndrome. Such tests include blood tests; the nuchal translucency test; and other ultrasound screens which look for certain abnormal features ('markers') in the fetus (such as absent nasal bone for example). When screening tests suggest a high risk of the fetus having Down syndrome, the woman is offered a diagnosis test (amniocentesis, CVS, PUB). Because screening tests only asses the risk, and while being accurate (and continuing to improve with medical advances) they still miss cases, some women, especially those of advanced age, choose to go directly to amniocentesis or CVS. These tests, however, are invasive, and, as such, carry a small risk of miscarriage (about 0.5% for amniocentesis and 1% for CVS).

A non-invasive prenatal test, MaterniT21, detected Down syndrome based on fetal DNA in a sample of the mother's blood in 209 of 212 cases (98.6%). The International Society for Prenatal Diagnosis finds that this is an advanced screening test which may be of use, in conjunction with genetic counseling, in high-risk cases based upon existing screening strategies. While effective in the diagnosis of Down syndrome, it cannot assess other conditions which can be detected by invasive testing; (for pregnant women who are screen-positive using current screening protocols, Down syndrome represents about half of the fetal chromosomal abnormalities identified through amniocentesis and CVS).

Table 1: First and second trimester Down syndrome screens
Screen When performed (weeks gestation) Detection rate False positive rate Description
Quad screen 15–20 81% 5% This test measures the maternal serum alpha feto protein (a fetal liver protein), estriol (a pregnancy hormone), human chorionic gonadotropin (hCG, a pregnancy hormone), and inhibin-Alpha (INHA).
Nuchal translucency/free beta/PAPPA screen (aka "1st Trimester Combined Test") 10–13.5 85% 5% Uses ultrasound to measure Nuchal Translucency in addition to the freeBeta hCG and PAPPA (pregnancy-associated plasma protein A). NIH has confirmed that this first trimester test is more accurate than second trimester screening methods.
Integrated test 10-13.5 and 15–20 95% 5% The integrated test uses measurements from both the 1st trimester combined test and the 2nd trimester quad test to yield a more accurate screening result. Because all of these tests are dependent on accurate calculation of the gestational age of the fetus, the real-world false-positive rate is >5% and may be closer to 7.5%.

Read more about this topic:  Down Syndrome

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