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Prenatal testing
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==== Maternal serum screening ==== First-trimester maternal serum screening can check levels of free Ξ²-[[human chorionic gonadotropin|hCG]], [[Pregnancy-associated plasma protein A|PAPP-A]], intact or beta hCG, or h-hCG in the woman's [[blood plasma|serum]], and combine these with the measurement of [[nuchal translucency]] (NT). Some institutions also look for the presence of a fetal nasalbone on the ultrasound.{{cn|date=February 2025}} Second-trimester maternal serum screening ([[AFP screening]], triple screen, quad screen, or penta screen) can check levels of [[alpha fetoprotein]], Ξ²-[[human chorionic gonadotropin|hCG]], [[inhibin]]-A, [[estriol]], and h-hCG (hyperglycosolated hCG) in the woman's [[blood plasma|serum]].{{cn|date=February 2025}} The [[triple test]] measures [[Blood serum|serum]] levels of [[Alpha-fetoprotein|AFP]], [[estriol]], and [[beta-hCG]], with a 70% [[sensitivity and specificity|sensitivity]] and 5% [[false-positive]] rate. It is complemented in some regions of the United States, as the ''Quad test'' (adding inhibin A to the panel, resulting in an 81% sensitivity and 5% false-positive rate for detecting [[Down syndrome]] when taken at 15β18 weeks of [[Gestational age (obstetrics)|gestational age]]).<ref>{{cite journal |vauthors=Lao MR, Calhoun BC, Bracero LA, Wang Y, Seybold DJ, Broce M, Hatjis CG |year=2009 |title=The ability of the quadruple test to predict adverse perinatal outcomes in a high-risk obstetric population |journal=[[Journal of Medical Screening]] |volume=16 |issue=2 |pages=55β9 |doi=10.1258/jms.2009.009017 |pmid=19564516 |s2cid=23214929}}</ref> The biomarkers [[PAPP-A]] and Ξ²-[[human chorionic gonadotropin|hCG]] seem to be altered for pregnancies resulting from ICSI, causing a higher false-positive rate. Correction factors have been developed and should be used when screening for Down's syndrome in singleton pregnancies after ICSI,<ref name="Gjerris2012">{{cite journal |vauthors=Gjerris AC, Tabor A, Loft A, Christiansen M, Pinborg A |date=July 2012 |title=First trimester prenatal screening among women pregnant after IVF/ICSI |journal=Human Reproduction Update |volume=18 |issue=4 |pages=350β9 |doi=10.1093/humupd/dms010 |pmid=22523111 |doi-access=free}}</ref> but in twin pregnancies such correction factors have not been fully elucidated.<ref name="Gjerris2012" /> In [[vanishing twin]] pregnancies with a second gestational sac with a dead fetus, first-trimester screening should be based solely on the maternal age and the nuchal translucency scan as biomarkers are altered in these cases.<ref name="Gjerris2012" />
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