First trimester screening is the current standard of care for pregnant women in the United States. Women with a high screening risk for trisomy then have invasive testing, which carries a risk of miscarriage, to definitively determine if the fetus has trisomy. Because of the high false negative rate of the first trimester screening, an unacceptable number of trisomic fetuses are not detected. Moreover, because of the high false positive rate, an unacceptable number of women undergo invasive follow up testing. Additional screening tests are needed that combine a high sensitivity, a low false positive rate, and minimal or no risk to the fetus.
