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Progress in Prenatal Diagnosis

Today it is possible to diagnose many genetic diseases and congenital defects while the fetus is still in the mother’s uterus. One procedure uses high frequency sound waves, or ultrasound, to determine the overall condition of the fetus and to confirm its age. Because it can be performed without the withdrawal of any fluid or tissue from the uterus or the fetus, ultrasound is called a noninvasive procedure. This test usually is performed during the fourth month of pregnancy. At this time major developmental abnormalities, such as extra or misplaced limbs, can be detected and the sex of the fetus can be determined.

Often ultrasound is used in conjunction with more invasive tests. Amniocentesis, which came into prominence in the 1970s, allows physicians to check for chromosomal abnormalities and certain enzymes disorders. For this test the doctor first uses ultrasound to determine the position of the fetus and the location of the placenta. Next a long thin needle is inserted through the mother’s abdomen into her uterus. About 10 ml of amniotic fluid is withdrawn, along with cells that have been sloughed off from the fetus. These cells are cultured for several weeks and then analyzed. The amniotic fluid is checked for increased levels of a substance called alphafetoprotein (AFP). High levels of AFP are found in such disorders as spina bifida. Amniocentesis cannot be performed until about the sixteenth week of pregnancy because there is not enough room in the amniotic sac to allow the safe withdrawal of fluid. Amniocentesis is quite accurate and safe. The risk of spontaneous abortion after the procedure is less than 0.5 percent (1 in 200).
Chrionic villi sampling (CVS) was first introduced in the early 1980s. CVS can be performed as early as the ninth week of pregnancy. A catheter is inserted through the vagina into the uterus and directed by ultrasound to a region called the chorion frondosum, where there are many chorionic villi. Some villi are withdrawn by suction through the catheter. As the cells in the villi are growing and dividing, they can be analyzed quickly. Results from chromosome studies are available in a few days and results from enzyme studies in about one week. Because no amniotic fluid is withdrawn in this procedure. AFP analysis cannot be performed; otherwise, diagnoses from CVS are as accurate as those from amniocentesis.  CVS carries a higher risk of spontaneous abortion-about 1 to 2 percent than amniocentesis does, but it allows earlier diagnosis of many defects.

Prenatal diagnosis is recommended routinely for women 35 years of age and older because the risk of chromosomal disorders increases with advanced maternal age. Prenatal diagnosis also is recommended for women who already have a child with a genetic disorder or who have a history of a genetic disorder in the family of either parent. All detectable chromosomal disorders and more than 200 single gene disorders now are diagnosable using amniocenteses and CVS.
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