Editing Embryo transfer (section)

== Fresh versus frozen ==
Embryos can be either "fresh" from fertilized egg cells of the same [[menstrual cycle]], or "frozen", that is they have been generated in a preceding cycle and undergone [[embryo cryopreservation]], and are thawed just prior to the transfer, which is then termed "frozen embryo transfer" (FET). The outcome from using cryopreserved embryos has uniformly been positive with no increase in birth defects or development abnormalities,<ref>{{cite web|url=http://www.givf.com/fertility/embryofreezing.cfm |archive-url=https://archive.today/20121206043620/http://www.givf.com/fertility/embryofreezing.shtml |archive-date=6 December 2012 |title=Genetics & IVF Institute |publisher=Givf.com |url-status=dead |access-date=22 September 2016 |df=dmy }}</ref> also between fresh versus frozen eggs used for [[intracytoplasmic sperm injection]] (ICSI).<ref name=Wennerholm2009>{{cite journal | vauthors = Wennerholm UB, Söderström-Anttila V, Bergh C, Aittomäki K, Hazekamp J, Nygren KG, Selbing A, Loft A | title = Children born after cryopreservation of embryos or oocytes: a systematic review of outcome data | journal = Human Reproduction | volume = 24 | issue = 9 | pages = 2158–72 | date = September 2009 | pmid = 19458318 | doi = 10.1093/humrep/dep125 | doi-access = free }}</ref> In fact, [[pregnancy rate]]s are increased following FET, and perinatal outcomes are less affected, compared to embryo transfer in the same cycle as [[controlled ovarian hyperstimulation|ovarian hyperstimulation]] was performed.<ref name="EvansHannan2014">{{cite journal | vauthors = Evans J, Hannan NJ, Edgell TA, Vollenhoven BJ, Lutjen PJ, Osianlis T, Salamonsen LA, Rombauts LJ | title = Fresh versus frozen embryo transfer: backing clinical decisions with scientific and clinical evidence | journal = Human Reproduction Update | volume = 20 | issue = 6 | pages = 808–21 | date = 2014 | pmid = 24916455 | doi = 10.1093/humupd/dmu027 | doi-access = free }}</ref> The [[endometrium]] is believed to not be optimally prepared for [[Implantation (human embryo)|implantation]] following ovarian hyperstimulation, and therefore frozen embryo transfer avails for a separate cycle to focus on optimizing the chances of successful implantation.<ref name="EvansHannan2014" /> Children born from [[Vitrification in cryopreservation|vitrified]] [[blastocysts]] have significantly higher [[birthweight]] than those born from non-frozen blastocysts.<ref>{{cite journal | vauthors = Wikland M, Hardarson T, Hillensjö T, Westin C, Westlander G, Wood M, Wennerholm UB | title = Obstetric outcomes after transfer of vitrified blastocysts | journal = Human Reproduction | volume = 25 | issue = 7 | pages = 1699–707 | date = July 2010 | pmid = 20472913 | doi = 10.1093/humrep/deq117 | doi-access = free }}</ref> When transferring a frozen-thawed oocyte, the chance of pregnancy is essentially the same whether it is transferred in a natural cycle or one with [[ovulation induction]].<ref name=Farquhar2014 />

There is probably little or no difference between FET and fresh embryo transfers in terms of live birth rate and ongoing pregnancy rate and the risk of [[ovarian hyperstimulation syndrome]] may be less using the "freeze all" strategy.<ref name=":0">{{Cite journal|last1=Zaat|first1=Tjitske|last2=Zagers|first2=Miriam|last3=Mol|first3=Femke|last4=Goddijn|first4=Mariëtte|last5=van Wely|first5=Madelon|last6=Mastenbroek|first6=Sebastiaan|date=2021-02-04|title=Fresh versus frozen embryo transfers in assisted reproduction|journal=The Cochrane Database of Systematic Reviews|volume=2021|issue=2|pages=CD011184|doi=10.1002/14651858.CD011184.pub3|issn=1469-493X|pmid=33539543|pmc=8095009}}</ref> The risk of having a large-for-gestational-age baby and higher birth rate, in addition to [[maternal hypertensive disorders]] of pregnancy may be increased using a "freeze all" strategy.<ref name=":0" />