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.
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.