I think you're talking about amniocentesis, right?
99% accuracy
0.5 to 1% fetal loss rate
0.32 to 1.6% chance that the cells won't grow after early amniocentesis
1 in 700 chance of nongrowth with second trimester amniocentesis (which
means the test must be repeated)
An inadequate amount of fluid or contamination of the sample also
necessitates a repeat test
Decaying cells are used, which often makes it difficult to determine whether
abnormalities are truly present
Errors can cause false-positive results
Maternal cells may be confused with fetal cells, although an experienced lab
should have a minimum of such accidents
Risks during actual procedure include damage from the needle to the baby,
placenta, or cord; tearing the uterus; infection; hemorrhage; and the
precipitation of a miscarriage
If test is negative, the baby may still suffer from problems that the exam
cannot (or failed to) detect
Delayed risks include a documented increase in newborn respiratory distress
(2.1 relative risk) and pneumonia (2.5 relative risk) after midtrimester
amniocentesis
It was found that pulmonary complications in 6.6% of babies subjected to
early amniocentesis, 1.6% of whom had respiratory distress syndrome
Reference: Frye, Anne, Understanding Diagnostic Tests in the Childbearing
Year, 7th ed. (Portland: Labrys Press, 2007), p.999-1000.
Best wishes.
