Editing Down syndrome (section)

====Efficacy====
For combinations of ultrasonography and non-genetic blood tests, screening in both the first and second trimesters is better than just screening in the first trimester.<ref name=ACOG2007/> Common screening techniques in use are able to pick up 90–95% of cases, with a false-positive rate of 2–5%.<ref name=Can2012>{{cite journal | vauthors = Canick J | title = Prenatal screening for trisomy 21: recent advances and guidelines | journal = Clinical Chemistry and Laboratory Medicine | volume = 50 | issue = 6 | pages = 1003–1008 | date = June 2012 | pmid = 21790505 | doi = 10.1515/cclm.2011.671 | s2cid = 37417471 }}</ref> If Down syndrome occurs in one in 500 pregnancies with a 90% detection rate and the test used has a 5% false-positive rate, of 28 women who test positive on screening, only one will have a fetus with Down syndrome confirmed. If the screening test has a 2% false-positive rate, this means of 11 women who test positive on screening, only one will have a fetus with Down syndrome.<ref name=Can2012/>