Подпись:We have already discussed amniocentesis, which identifies chromosomal abnormalities in the 16 th to 17 th weeks of pregnancy. But many women elect to undergo a chorionic villus sampling (CVS) test, which can be done between the 10th and 15th weeks. In this procedure, a sliver of tissue from the chorion (the tissue that develops into the pla­centa) is removed and checked for chromosomal abnormalities.

Подпись:Подпись:Подпись: Age of MotherПодпись:Подпись: 20 1 in 526 25 1 in 476 30 1 in 384 35 1 in 192 40 1 in 66 45 1 in 21 49 1 in 8 Подпись:Подпись:Подпись: ©LWA-Dann Tardif/CorbisПодпись:Chromosomal AbnormalitiesПодпись: Down syndrome, a chromosomal defect, can cause mental retardation and the characteristics of slanted eyes and flat face.There are drawbacks to CVS, however. Some CVS tests will result in a false positive because the sample may have a different chromosomal composi­tion than the fetus. In addition, there is an increased risk of miscarriage and limb reduction/deformities with this test. However, the risk to fetal limbs is re­duced when CVS is performed after 9 weeks gestation. As a result, CVS at most institutions is performed af­ter the 10th week (Mastroiacovo, 1992). Overall, the CVS test is viewed as the “gold standard” sampling method for genetic testing because it can be per­formed earlier in pregnancy than other tests (Brambati & Tului, 2005).

Another test, the maternal-serum alpha-fetoprotein screening (MSAFP), is used to detect defects such as spina bifida (SPY-na BIF-id-uh) or anencephaly (an-en-SEH-fuh-lee). MSAFP is a sim­ple blood test done between the 16th and 18th weeks of pregnancy. However, due to this test’s high frequency of false positive test results, second screenings are recommended. Finally, a blood sample can also be collected from the umbilical cord anytime after 18 weeks of pregnancy, and a chromosome analysis can be done (Berkow et al., 2000). All of these tests allow healthcare providers to detect fetal abnormalities.

The risk of chromosomal abnormalities increases as a woman ages, and chromoso­mal abnormalities can result in many different problems (see Table 12.3). Sometimes physicians are certain of where the chromosomal problem lies and how it will manifest itself; at other times, they just don’t know. The most common chromosomal abnormal­ity appears in the 21st chromosome and is known as Down syndrome. In Down syn­drome, an extra chromosome has been added to the 21st chromosome; although most of us have 46 chromosomes (23 from each parent), a person with Down syndrome has 47. Down syndrome occurs in 1 out of every 1,000 live births.

Down syndrome is often blamed on ova that have aged. For that reason, older par­ents are at greater risk for Down syndrome children than younger parents. But it can also be due to deficient sperm. It is estimated that 25% of Down syndrome cases can be traced to defective sperm. A child with Down syndrome often exhibits low muscle tone, a flat facial profile, slanted eyes, mental retardation, and an enlarged tongue. There are also many long-term health concerns including increased risk of congenital heart defects, respiratory infections, Alzheimer’s, and leukemia (National Down Syndrome Society,

2005) . In 2005, a study on over 38,000 U. S. women found that a first-trimester screen­ing test, combining a blood test with an ultrasound, can detect Down syndrome 11 weeks after conception (Malone et al., 2005). This test was based on research from a $15 million, 8-year study published in 2005. Although research has been in progress on first – trimester testing (Nicolaides et al., 2005; Orlandi et al., 2005), testing for Down syn­drome typically had involved waiting for a CVS (10th – to 15th-week) or amniocentisis (16th – to 17th-week) test.