PRENATAL DIAGNOSTIC TESTS

 PRENATAL DIAGNOSTIC TESTS

One choice open to prospective mothers is the option to undergo prenatal testing. A number of tests can indicate whether a fetus is developing normally, including ultrasound sonography, fetal MRI, chorionic villus sampling, amniocentesis, maternal blood screening, and noninvasive prenatal diagnosis (Lenzi & Johnson, 2008).

An ultrasound test is often conducted seven weeks into a pregnancy and at various times later in pregnancy (Cignini & others, 2010). Ultrasound sonography is a prenatal medical procedure in which high-frequency sound waves are directed into the pregnant woman’s abdomen. The echo from the sounds is transformed into a visual representation of the fetus’s inner structures. This technique can detect many structural abnormalities in the fetus, including microencephaly, a form of mental retardation involving an abnormally small brain; it can also determine the number of fetuses and give clues to the baby’s sex (Gerards & others, 2008). There is virtually no risk to the woman or fetus in this test.

The development of brain-imaging techniques has led to increasing use of fetal MRI to diagnose fetal malformations (Daltro & others, 2010; Duczkowska & others, 2010) (see Figure 2.7). MRI stands for magnetic resonance imaging and uses a powerful magnet and radio images to generate detailed images of the body’s organs and structures. Currently, ultrasound is still the first choice in fetal screening, but fetal MRI can provide more detailed images than ultrasound. In many instances, ultrasound will indicate a possible abnormality and then fetal MRI will be used to obtain a clearer, more detailed image (Obenauer & Maestre, 2008). Among the fetal malformations that fetal MRI may be able to detect better than ultrasound sonography are certain abnormalities of the central nervous system, chest, gastrointestinal tract, genital/urinary system, and placenta (Baysinger, 2010; Panigraphy, Borzaga, & Blumi, 2010; Weston, 2010), motivation to terminate a pregnancy (Benn & Chapman. 2010).

At some point between the 10th and 12th weeks of pregnancy, chorionic villus sampling may be used to detect genetic defects and chromosomal abnormalities such as those discussed in the previous section. Chorionic villus sampling (CVS) is a prenatal medical procedure in which a small sample of the placenta (the vascular organ that links the fetus to the mother’s uterus) is removed. Diagnosis takes about 10 days. There is a small risk of limb deformity when CVS is used.

Between the 15th and 18th weeks of pregnancy, amniocentesis may be performed. Amniocentesis is a prenatal medical procedure in which a sample of amniotic fluid is withdrawn by syringe and tested for chromosomal or metabolic disorders (Nagel & others, 2007). The amnionic fluid is found within the amnion, a thin sac in which the embryo is suspended. Ultrasound sonography is often used during amniocentesis so that the syringe can be placed precisely. The later in the pregnancy amniocentesis is performed, the better its diagnostic potential. The earlier it is performed, the more useful it is in deciding how to handle a pregnancy. It may take two weeks for enough cells to grow and amniocentesis test results to be obtained. Amniocentesis brings a small risk of miscarriage: about 1 woman in every 200 to 300 miscarries after the procedure.

Both amniocentesis and chorionic villus sampling provide valuable information about the presence of birth defects, but they also raise difficult issues for parents about whether an abortion should be obtained if birth defects are present (Quadrelli & others, 2007; Zhang & others, 2010). Chorionic villus sampling allows parents to make a decision sooner, near the end of the first 12 weeks of pregnancy, when abortion is safer and less traumatic than later. Although earlier reports indicated that chorionic villus sampling brings a slightly higher risk of pregnancy loss than amniocentesis, a recent U.S. study of more than 40,000 pregnancies found that loss rates for CVS decreased over the period from 1998 to 2003 and that there is no longer a difference in pregnancy loss risk between CVS and amniocentesis (Caughey, Hopkins, & Norton, 2006).

During the 16th to 18th weeks of pregnancy, maternal blood screening may be performed. Maternal blood screening identifies pregnancies that have an elevated risk for birth defects such as spina bififida (a defect in the spinal cord) and Down syndrome (Bustamante-Aragones & others, 2010). The current blood test is called the triple screen because it measures three substances in the mother’s blood. After an abnormal triple screen result, the next step is usually an ultrasound examination. If an ultrasound does not explain the abnormal triple screen results, amniocentesis is typically used.

Noninvasive prenatal diagnosis (NIPD) is increasingly being explored as an alternative to procedures such as chorionic villus sampling and amniocentesis (Susman & others, 2010). At this point, NIPD has mainly focused on the isolation and examination of fetal cells circulating in the mother’s blood and analysis of cell-free fetal DNA in maternal plasma (Prakash, Powell, & Geva, 2010).

Researchers already have used NIPD to successfully test for genes inherited from a father that cause cystic fibrosis and Huntington disease. They also are exploring the potential for using NIPD to identify a baby’s sex as early as five weeks after conception and to diagnose Down syndrome (Avent & others, 2008). Being able to detect an offspring’s sex and various diseases and defects so early raises ethical concerns about couples’ motivation to terminate a pregnancy (Benn & Chapman, 2010).