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Page: Prenatal Screening
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Pregnant women can be screened for various complications in their pregnancy. Many standard prenatal screens can discover Down syndrome. Genetic counseling along with genetic testing, such as amniocentesis, chorionic villus sampling (CVS), or percutaneous umbilical blood sampling (PUBS) are usually offered to families who may have an increased chance of having a child with Down syndrome, or where normal prenatal exams indicate possible problems. Genetic screens are often performed on pregnant women older than 30 or 35.
Amniocentesis and CVS are considered invasive procedures, in that they involve inserting instruments into the uterus, and therefore carry a small risk of causing fetal injury or miscarriage. There are several common non-invasive screens that can indicate a fetus with Down syndrome. These are normally performed in the late first trimester or early second trimester. Due to the nature of screens, each has a significant chance of a false positive, suggesting a fetus with Down syndrome when, in fact, the fetus does not have this genetic abnormality. Screen positives must be verified before a Down syndrome diagnosis is made. Common screening procedures for Down syndrome are given in Table 1.
Table 1: Common first and second trimester Down syndrome screens Screen When performed (weeks gestation) Detection rate False positive rate Description
Triple screen 15–20 75% 8.5% This test measures the maternal serum alpha feto protein (a fetal liver protein), estriol (a pregnancy hormone), and human chorionic gonadotropin (hCG, a pregnancy hormone).
Quad screen 15–20 79% 7.5% 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 high inhibin-Alpha (INHA).
AFP/free beta screen 13–22 80% 2.8% This test measures the alpha feto protein, produced by the fetus, and free beta hCG, produced by the placenta.
Nuchal translucency/free beta/PAPPA screen 10–13.5 91% 5% Uses ultrasound to measure Nuchal Translucency in addition to the freeBeta hCG and PAPPA (pregnancy-associate plasma protein A, Mendelian Inheritance in Man (OMIM) 176385). NIH has confirmed that this first trimester test is more accurate than second trimester screening methods.
Even with the best non-invasive screens, the detection rate is 90%–95% and the rate of false positive is 2%–5%. False positives can be caused by undetected multiple fetuses (very rare with the ultrasound tests), incorrect date of pregnancy, or normal variation in the proteins.
Confirmation of screen positive is normally accomplished with amniocentesis. This is an invasive procedure and involves taking amniotic fluid from the mother and identifying fetal cells. The lab work can take several weeks but will detect over 99.8% of all numerical chromosomal problems with a very low false positive rate.
Due to the low incidence of Down syndrome, a vast majority of early screen positives are false. Since false positives typically prompt an amniocentesis to confirm the result, and the amniocentesis carries a small risk of inducing miscarriage, there is a slight risk of miscarrying a healthy fetus. (The added miscarriage risk from an amniocentesis is traditionally quoted as 0.5%, but recent studies suggest that it may be considerably smaller (0.06% with a 95% CI of 0 to 0.5%).)
A 2002 literature review of elective abortion rates found that 91–93% of pregnancies with a diagnosis of Down syndrome were terminated.[22] Physicians and ethicists are concerned about the ethical ramifications, with some commentators calling it "eugenics by abortion". Many members of the disability rights movement "believe that public support for prenatal diagnosis and abortion based on disability contravenes the movement's basic philosophy and goals."
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