Thanks to the wonders of modern genetics, it is possible to learn many things about the unborn fetus. Everything from the child's sex, to potential genetic diseases can be diagnosed as early as 6 or 7 weeks after conception. A modern tool of p renatal diagnosis is Chorionic Villus Sampling, or CVS.

Background: Amniocentesis

As early as the 1930's, invasive techniques were being used to analyze the developing fetus in the womb of pregnant mothers. By the 1950's, amniocentesis was being used to determine blood type compatibility between mothers and their children. In 1967 , the first case of the use of amniocentesis to test for genetic disease was reported. The development of ultrasound has greatly decreased the risk of the test. The procedure involves the insertion of a long needle into the uterus and amniotic sac to sa mple the amniotic fluid which is surrounding the develop fetus. This fluid is rich in the fetus's genetic material, and can be analyzed for diverse amounts of genetic material. The test is usually given at about the 16^th week of pregnancy, an d can take several weeks to process the sample.

Newer procedures have been developed to analyze the fetus's genetic material that can occur earlier and can be processed faster than amniocentesis. One, known as Percutaneous Umbilical Blood Sampling, or PUBS, involves taking blood from the umbilical vein for sampling. Another, examined here, is known as Chorionic Villus Sampling, or CVS.

CVS: A modern alternative to amniocentesis.

CVS was developed in 1968 in Denmark, but was soon abandoned due to higher risk of miscarriage. The procedure involves removing a small sample of villi, or finger like projections, from the surface of the placenta, or chorion. Several methods of obta ining these cells have been attempted, but many were abandoned due to risk to the fetus and mother. The test was revived in 1982 in the former USSR as a method of testing for genetic disease in the fetus, and safer, more refined sampling procedures soon followed. Currently, the use of CVS is a common, safe procedure, whose risk level is on the order of that of amniocentesis.

CVS usually occurs between the ninth and eleventh week of pregnancy, though it can be performed at nearly any stage. This greater flexibility is an obvious advantage over amniocentesis, as a much earlier diagnosis of potential problems is possible. Two methods of obtaining chorionic villi are commonly used today. The first involves the use of a small cannula inserted through the cervix. The second involves a procedure very similar to amniocentesis, where a long needle is inserted through the mothe r's abdomen to obtain a sample. In both cases, ultrasound is commonly used to guide the doctor in obtaining the sample. Each test has advantages and disadvantages, but in the hands of an experienced doctor, both are very low risk.

Overall, CVS has a higher failure rate than amniocentesis. Up to 10% of women need additional testing after their first CVS, while amniocentesis has a failure rate of lower than 1%. However, if successful, CVS provides much earlier results, allowing prospective parents more time to make the difficult decisions associated with potential genetic problems. Also, there is a 0.6 to 0.8 % higher chance of miscarriage due to CVS than due to amniocentesis. However, many of these disadvantages are countered by the much earlier results afforded by CVS.