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This is a non-invasive procedure. The developing embryo can first be visualized at about 6 weeks gestation. Recognition of the major internal organs and extremities to determine if any are abnormal can best be accomplished between 16 to 20 weeks gestation. Although an ultrasound examination can be quite useful to determine the size and position of the fetus, the size and position of the placenta, the amount of amniotic fluid, and the appearance of fetal anatomy, there are limitations to this procedure. Subtle abnormalities may not be detected until later in pregnancy, or may not be detected at all. A good example of this is Down's syndrome (trisomy 21) where the morphologic abnormalities are often not marked, but only subtle, such as nuchal thickening.

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Prenatal Diagnosis - Overview


Maternal blood sampling for fetal blood cells
"Several features on ultrasound scans that doctors thought were warning signs of Down syndrome are almost always harmless, leading many women with normal fetuses to undergo unnecessary amniocentesis, researchers say....

Researchers looked at seven markers sometimes spotted on ultrasound scans that are thought to increase the risk of Down. Only one of those markers--a shaded area suggesting a thickening at the back of the neck--was found to be a reliable indicator. The six other markers were certain brain cysts, shortened thigh and upper-arm bones, bright spots on the bowel or heart, and high fluid levels in the kidneys. About 10 percent to 14 percent of pregnant women have one of these markers, but fewer than 1 percent have Down babies. Ultrasounds that show an obvious structural abnormality, such as heart defects that are common in Down syndrome, should not be discounted.

In 1998, the latest year for which figures are available, 112,778 amniocenteses and 2.5 million ultrasounds were performed in the United States. The risks of miscarriage from amniocentesis are greater than the risks that a woman with normal blood tests but one of the ultrasound markers will have a Down syndrome baby, according to the analysis." emphasis mine

- AP wire service report, Nov. 23, 2000

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