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Screening for Neural Tube Defects as a Model

The available tests and medical standards for detecting specific genetic diseases are changing rapidly. Rather than attempting to discuss each test and disease, this chapter identifies commonalties in counseling and testing through a discussion of a small number of diseases. Currently, maternal serum alpha-fetoprotein (MSAFP) is the only screening test for genetic diseases recommended for all women.[182] It provides a good model for genetic counseling because it is a disease with both a hereditary component and a random component that defies easy separations into risk groups. Neural tube defects are common, occurring in 1 or 2 per 1000 births among couples with no history. With one affected parent, the risk rises to 5 percent of births, rising further to 6 to 10 percent if the couple has two previous affected children. Despite this genetic link, 90 to 95 percent of the cases are in families with no previous history, prompting the recommendation that the screening test be offered to all pregnant women.

When first made, this recommendation was controversial because the test for MSAFP is quite sensitive but not very specific. There was concern that women would be frightened into aborting fetuses on the basis of the preliminary screening test. This led to specific standards of practice that stress the importance of the entire process of counseling and testing:

The successful implementation of a screening program for MSAFP should include patient education, accurate and prompt laboratory testing, competent counseling and support services, access to consultants for sonography, and complex prenatal diagnosis, as well as available options for pregnancy termination. Success is further dependent on the proper coordination of these components, all of which must function within a relatively short time span from screening to decision-making. Missing components or malfunctions could result in unnecessary anxiety for the patients, as well as improper diagnoses that could lead to unnecessary termination of pregnancy or other serious errors in judgment.[183]

The coordination of the various components is critical because it is recommended that the test not be performed until 16 weeks of gestation. If the test indicates a sufficiently increased level of MSAFP, the patient should be offered a second test a week or two later, if time permits. Otherwise ultrasound should be used to correct the gestational age, check for multiple gestation, and, if possible, identify a neural tube defect. Among patients with two high MSAFP levels (or one low one) slightly over half will have a singleton fetus at the appropriate gestational age without an apparent anomaly. These patients should be offered amniocentesis. Of the patients undergoing amniocentesis, one to two will have significantly increased amniotic fluid AFP that indicates a high probability of a fetus with a serious abnormality.[184]

At this point further tests can be done to identify the specific defect, but this will be impossible in some cases. If a defect is confirmed, the patient must decide whether she wants an abortion. Many patients choose to abort the fetus. Some may be willing to accept a child with spina bifida but be unwilling to carry a fetus with anencephaly. The woman with an elevated amnionic AFP but no identifiable defect has the more difficult decision. She should be given full information and an opportunity to seek in-depth counseling before making her decision. Once the process of recommending the initial test begins, every following step must be planned carefully and executed. It is critical that the patient be carefully tracked to ensure that there are no delays that can push the abortion into the third trimester, with the attendant medical and legal complications.

[182]ACOG Technical Bulletin 154: Prenatal Detection of Neural Tube Defects. April 1991.

[183]ACOG Technical Bulletin 154: Prenatal Detection of Neural Tube Defects. April 1991.

[184]ACOG Technical Bulletin 154: Prenatal Detection of Neural Tube Defects. April 1991.

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