Prenatal Tests

Several types of tests performed on pregnant women can identify increased risk of carrying a fetus with a chromosomal problem or actually detect the abnormal chromosomes (fig. 24.15) A blood test performed during the fifteenth week of pregnancy detects levels of maternal serum markers (specifically, alpha fetoprotein, a form of estrogen, and human chorionic gonadotropin) that can indicate the underdeveloped liver that is a symptom of certain trisomies, including Down syndrome. Often called the "triple test" or "AFP test," screening maternal serum markers is routine in the management of pregnancy.

A maternal serum marker pattern indicating increased risk is typically followed by amniocentesis, in which a physician inserts a needle into the amniotic sac and withdraws about 5 milliliters of fluid. Fetal fibro-blasts in the sample are cultured and a karyotype constructed, which reveals extra, missing, or translocated chromosomes or smaller anomalies. However, amniocen-tesis cannot reveal single gene defects. Because amniocentesis has a risk of about 0.5% of being followed by miscarriage, it is typically performed on women whose risk of carrying an affected fetus is greater than this, which includes all women over age thirty-five and those with a family history of a chromosomal disorder on either parent's side.

Couples who have already had a child with a chromosome abnormality can elect to have chorionic villus sampling (CVS), a test that has the advantage of being performed as early as the tenth week from conception, but carries a higher risk of being followed by miscarriage than does amniocentesis. In CVS, a physician samples chorionic villus cells through the cervix. The basis of the test is that, theoretically, these cells are genetically identical to fetal cells because they too descend from the

Clinical Application

Down Syndrome

The most common autosomal aneuploid is trisomy 21, an extra chromosome 21. The characteristic slanted eyes and flat face of affected individuals prompted Sir John Langdon Haydon Down to coin the inaccurate term "mongolism" when he described the syndrome in 1886. As the medical superintendent of a facility for the profoundly mentally retarded, Down noted that about 10% of his patients resembled people of the Mongolian race. The resemblance is coincidental. Males and females of all races can have the syndrome.

A person with Down syndrome (either trisomy or translocation) is short and has straight, sparse hair and a tongue protruding through thick lips. The face has other telltale characteristics, including upward slanting eyes with "epicanthal" skin folds in the inner corners and abnormally shaped ears. The hands have an abnormal pattern of creases, the joints are loose, and reflexes and muscle tone are poor. Developmental milestones (such as sitting, standing, and walking) are slow, and toilet training may take several years. Intelligence varies greatly, from profound mental retardation to following simple directions, reading and using a computer. People with the syndrome have graduated from college.

Down syndrome (either type) is associated with many physical problems. Nearly 50% of affected people die before their first birthdays, often of heart or kidney defects, or of infections, which can be severe due to a suppressed immune system. Blockages in the digestive system are common and must be corrected surgically shortly after birth. An affected child is 15 times more likely to

Wendy Weisz has trisomy 21 Down syndrome. She enjoys studying art at Cuyahoga Community College.

fertilized ovum. However, sometimes a mutation can occur in a villus cell only, or a fetal cell only, creating a false positive or false negative test result.

An experimental prenatal test, fetal cell sorting, is safer than amniocentesis or CVS because it samples only maternal blood, yet it provides the high accuracy of these tests. It is more accurate than measuring maternal serum markers. Fetal cell sorting separates out the rare fetal cells that normally cross the placenta and enter the woman's circulation; then a karyotype is constructed from the sampled cells. It can be performed early in pregnancy but so far is too costly to be widely implemented.

Table 24.1 summarizes the tests used to visualize fetal chromosomes as a window onto health. In the near future, DNA microarray analyses that screen for individual genes will likely replace these chromosome-detection tests, as described in the chapter opener.

O Why do deviations from the normal chromosome number of 46 affect health?

^9 Distinguish between polyploidy and aneuploidy.

^9 How do extra sets of chromosomes or extra individual chromosomes arise?

How are fetal chromosomes examined?

develop leukemia than a healthy child, but this is still a low figure. Prenatal testing cannot reveal how severely affected the individual will be.

About 25% of people with Down syndrome, trisomy 21 or translocation who live past age forty develop the fibers and tangles of amyloid protein in their brains that are also seen in the brains of people who have died of Alzheimer disease.

(The risk among the general population is 6%.) Both disorders seem to involve accelerated aging of part of the brain and accumulation of amyloid protein.

The likelihood of giving birth to a child with trisomy 21 Down syndrome increases dramatically with the age of the mother (Table 24A). However, 80% of children with trisomy 21 are born to women under age thirty-five, because younger women are more likely to be-

Risk of Trisomy 21 Increases with Maternal Age

Maternal Age

Trisomy 21 Risk

Risk for Any Aneuploid

Pregnancy Guide

Pregnancy Guide

A Beginner's Guide to Healthy Pregnancy. If you suspect, or know, that you are pregnant, we ho pe you have already visited your doctor. Presuming that you have confirmed your suspicions and that this is your first child, or that you wish to take better care of yourself d uring pregnancy than you did during your other pregnancies; you have come to the right place.

Get My Free Ebook


Post a comment