An imaging process that sends inaudible high-frequency sound waves through a transducer, a small device that the doctor moves slowly atop a pregnant woman's abdomen. The sound waves are bounced off fetal tissue and organs, translated by computer and then projected on a video screen, creating a blurred, translucent image of the fetus. The use of ultrasound is a key procedure in prenatal diagnosis as well as in selective termination of pregnancy.
During the second trimester--usually at 16 to 18 weeks, but earlier in some cases--a doctor inserts a needle into the amniotic fluid that surrounds the fetus and withdraws fluid. Fetal cells floating in the fluid can be grown in the laboratory and analyzed to detect evidence of genetic defects and to determine the fetus' sex. Results are available in about two weeks. Studies indicate that the accuracy of the test is 99.4%, while the risk of miscarriage or infection from the diagnosis is .2% or .3%. Most doctors do not routinely offer amniocentesis to patients younger than 35, unless they are at risk of having a child with a birth defect. After age 35, the risk of a woman's having a child with Down's syndrome, a genetic defect that leads to mental retardation, rises substantially. But many doctors consider "maternal anxiety" sufficient reason to offer the test to younger women.