Genetic Screening : Prenatal Peeks

by Food and Drug Administration (FDA)

(Page 4 of 4)

Ultrasound

In an ultrasound exam, a device called a transducer passed over the abdomen or inserted into the vagina bounces sound waves off the fetus, much like sonar locating a submarine. A computer converts the sound waves into an image. Many studies show it is very safe to mother and fetus.

Ultrasound can establish the date of conception, the presence of twins, and monitor development. By 8 weeks, an image resembling a lima bean with a pulsating blip in the middle is an assurance that a "viable fetus" with its blip of a heartbeat?is there.

By 15 weeks, a trained eye can discern major organs. While the parents happily count toes and fingers, a physician may measure the length of the leg bones or check facial features for signs of Down syndrome.

By 20 weeks, a penis?or lack of one?may be apparent. By 35 weeks, calcium deposits in the placenta, the organ linking mother to child, signal lung maturity. As the birth day nears, ultrasound reveals the fetal position.

Amniocentesis

Through amniocentesis, the amniotic fluid surrounding the fetus is sampled with a needle inserted into the woman's abdomen. Fetal cells floating in the fluid are grown and examined for chromosomal abnormalities, such as the extra chromosome 21 that causes Down syndrome. Biochemicals in the fluid also provide diagnostic clues to several inborn errors of metabolism. Approved since 1967, amniocentesis is offered to women over 35, the age when the risk of the procedure causing a miscarriage is equal to the risk of the woman carrying a fetus with a detectable chromosomal problem (this risk increases with age). Women younger than 35 may have the procedure if a relative has a detectable abnormality. Amniocentesis can rule out many disorders, but it cannot guarantee a healthy baby.

The major drawback of amniocentesis at present is that it cannot safely be performed until the 16th week of pregnancy, and it takes 10 days or longer for fetal cells to be cultured and results to be known. However, several medical centers are experimenting with performing amniocentesis as early as 12 weeks.

Another advance is the use of automated, computerized chromosome sorters that are programmed to scan for abnormalities. This replaces technicians cutting up photographs of chromosomes and arranging them into a standard chart, then searching visually for aberrations a time-consuming process. Yet another new approach to viewing chromosomes is "in situ hybridization," a technique that uses DNA probes (bits of DNA tagged with a chemical) to locate and highlight specific chromosomes with no need to culture them first. This approach, still for research use only, can identify the extra chromosome of Down syndrome in hours.

Chorionic Villus Sampling (CVS)

In CVS, recently approved by FDA, a catheter inserted through the vagina samples chorionic villi, finger-like structures that form the placenta by 10 weeks of prenatal development. Because villi cells descend from the fertilized egg, their chromosomes match those of the fetus.

The great advantage of CVS is that it can be performed as early as 8 weeks, and results are ready within days.

In the March 9, 1989, New England Journal of Medicine, George G. Rhoads, M.D., and co-workers at the National Institute of Child Health and Human Development reported on a seven-center study comparing CVS to amniocentesis. They conclude, "CVS is a safe and effective technique for the early prenatal diagnosis of cytogenetic abnormalities, but it probably entails a slightly higher risk of procedure failure and fetal loss than does amniocentesis." The risk of amniocentesis causing miscarriage is 0.5 percent; that of CVS is 1.3 percent. Miscarriages are more often associated when CVS is performed more than twice.

The Triple Test AFP-Plus

In 1975, scientists found a link between high levels of alpha-fetoprotein (AFP) in pregnant women's blood and a type of fetal abnormality called a neural tube defect, which includes spina bifida (an open spine) and anencephaly (lack of higher brain structures). The open lesions of such defects allow AFP to leak faster than normal from the fetus' liver into the mother's bloodstream, causing the elevated levels.

In 1984, studies linked too little of the substance to Down syndrome. After this discovery, measuring AFP at 15 weeks for use as a prenatal warning greatly expanded. This helps in spotting Down syndrome in women under 35, who would usually not have amniocentesis. Abnormal AFP levels may also be associated with other birth defects and with late miscarriage, low birth weight, toxemia (very high blood pressure in a woman in the last trimester of pregnancy), premature delivery, and other birth defects.

How can one substance show so much? "We think the abnormal readings reflect something wrong with the placenta," says Washington Hill, M.D., director of maternal-fetal medicine at the Creighton University School of Medicine in Omaha.

AFP testing has a very high false-positive rate, because the level of AFP can be thrown off by such factors as a miscalculated due date, obesity, twins, or being black or diabetic. In 1987, accuracy was improved by considering levels of human chorionic gonadotropin, too, which is high in Down syndrome. The recent addition of a third measurement unconjugated estriol may make readings even more accurate.

By indicating a low risk, the use of this "triple test" could spare some women over 35 from amniocentesis recommended only because of their age. However, this is a screening test, not a diagnostic test, cautions George J. Knight, Ph.D., of the Foundation for Blood Research in Scarborough, Maine, where the test was developed. Women with abnormal results must undergo more definitive tests, such as ultrasound and amniocentesis, before the diagnosis is considered final.

About the Author

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