Editing Prenatal testing (section)

==== Placental acellular DNA (pa-DNA) ====
Placental acellular (fetal cell-free) DNA testing (pa-DNA) allows for the detection of apoptotic placental cells and placental DNA circulating in maternal blood for the noninvasive diagnosis of fetal aneuploidy.<ref name="Ref2" /><ref name="Taglauer-2014">{{cite journal |vauthors=Taglauer ES, Wilkins-Haug L, Bianchi DW |date=February 2014 |title=Review: cell-free fetal DNA in the maternal circulation as an indication of placental health and disease |journal=Placenta |volume=35 Suppl |issue=Suppl |pages=S64-8 |doi=10.1016/j.placenta.2013.11.014 |pmc=4886648 |pmid=24388429}}</ref> A meta-analysis that investigated the success rate of using placental acellular DNA from maternal blood to screen for aneuploidies found that this technique detected trisomy 13 in 99% of the cases, trisomy 18 in 98% of the cases and trisomy 21 in 99% of the cases.<ref name="Taglauer-2014" /><ref>{{Cite journal |last1=Gil |first1=M. M. |last2=Galeva |first2=S. |last3=Jani |first3=J. |last4=Konstantinidou |first4=L. |last5=Akolekar |first5=R. |last6=Plana |first6=M. N. |last7=Nicolaides |first7=K. H. |date=June 2019 |title=Screening for trisomies by cfDNA testing of maternal blood in twin pregnancy: update of The Fetal Medicine Foundation results and meta-analysis |journal=Ultrasound in Obstetrics & Gynecology |volume=53 |issue=6 |pages=734–742 |doi=10.1002/uog.20284 |issn=1469-0705 |pmid=31165549 |doi-access=free|hdl=10641/1656 |hdl-access=free }}</ref> Failed tests using placental acellular DNA are more likely to occur in fetuses with trisomy 13 and trisomy 18 but not with trisomy 21.<ref>{{cite journal |vauthors=Revello R, Sarno L, Ispas A, Akolekar R, Nicolaides KH |date=June 2016 |title=Screening for trisomies by cell-free DNA testing of maternal blood: consequences of a failed result |journal=Ultrasound in Obstetrics & Gynecology |volume=47 |issue=6 |pages=698–704 |doi=10.1002/uog.15851 |pmid=26743020 |doi-access=free}}</ref> Previous studies found elevated levels of acellular placental DNA for trisomy 13 and 21 from maternal serum when compared to women with euploid pregnancies.<ref name="Wataganara-2003">{{Cite journal |last1=Wataganara |first1=T |last2=LeShane |first2=ES |display-authors=etal |year=2003 |title=Maternal serum cell-free fetal DNA levels are increased in cases of trisomy 13 but not trisomy 18. – PubMed – NCBI |journal=Human Genetics |language=en |volume=112 |issue=2 |pages=204–8 |doi=10.1007/s00439-002-0853-9 |pmid=12522563 |s2cid=9721963}}</ref><ref>{{cite journal |vauthors=Lee T, LeShane ES, Messerlian GM, Canick JA, Farina A, Heber WW, Bianchi DW |date=November 2002 |title=Down syndrome and cell-free fetal DNA in archived maternal serum |journal=American Journal of Obstetrics and Gynecology |volume=187 |issue=5 |pages=1217–21 |doi=10.1067/mob.2002.127462 |pmid=12439507 |s2cid=31311811}}</ref><ref>{{cite journal |display-authors=6 |vauthors=Lo YM, Lau TK, Zhang J, Leung TN, Chang AM, Hjelm NM, Elmes RS, Bianchi DW |date=October 1999 |title=Increased fetal DNA concentrations in the plasma of pregnant women carrying fetuses with trisomy 21 |journal=Clinical Chemistry |volume=45 |issue=10 |pages=1747–51 |doi=10.1093/clinchem/45.10.1747 |pmid=10508120 |doi-access=free}}</ref><ref name="Lo-2009">{{cite journal |vauthors=Lo YM |date=January 2009 |title=Noninvasive prenatal detection of fetal chromosomal aneuploidies by maternal plasma nucleic acid analysis: a review of the current state of the art |journal=BJOG |volume=116 |issue=2 |pages=152–7 |doi=10.1111/j.1471-0528.2008.02010.x |pmid=19076946 |s2cid=6946087}}</ref> However, an elevation of acellular DNA for trisomy 18 was not observed.<ref name="Wataganara-2003" /> Circulating placental nucleated cells comprise only three to six percent of maternal blood plasma DNA, reducing the detection rate of fetal developmental abnormalities.<ref name="Lo-2009" /> Two alternative approaches have been developed for the detection of fetal aneuploidy. The first involves the measuring of the [[Allele|allelic ratio]] of [[single nucleotide polymorphism]]s (SNPs) in the [[mRNA]] coding region in the [[placenta]]. The next approach is analyzing both maternal and placental DNA and looking for differences in the [[DNA methylation]] patterns.<ref name="www.acog.org-2" /><ref name="Lo-2009" />