Editing In vitro fertilisation (section)

===Final maturation induction===
{{Further|Final maturation induction}}
When the ovarian follicles have reached a certain degree of development, [[Final maturation induction|induction of final oocyte maturation]] is performed, generally by an injection of [[human chorionic gonadotropin]] (hCG). Commonly, this is known as the "trigger shot."<ref>{{cite web |url=http://infertility.about.com/od/infertilitytreatments/ss/ivf_treatment_5.htm |title=IVF Treatment Step Four: Final Oocyte |publisher=Infertility.about.com |access-date=22 May 2012 |archive-date=18 November 2012 |archive-url=https://web.archive.org/web/20121118205858/http://infertility.about.com/od/infertilitytreatments/ss/ivf_treatment_5.htm |url-status=dead }}</ref> hCG acts as an analogue of [[luteinising hormone]], and ovulation would occur between 38 and 40 hours after a single HCG injection,<ref>{{cite web | url = http://www.medscape.com/viewarticle/473515 | title = HCG Injection After Ovulation Induction With Clomiphene Citrate] | work = Medscape | vauthors = Kovacs P | date = 23 April 2004 }}</ref> but the egg retrieval is performed at a time usually between 34 and 36 hours after hCG injection, that is, just prior to when the follicles would rupture. This avails for scheduling the egg retrieval procedure at a time where the eggs are fully mature. HCG injection confers a risk of [[ovarian hyperstimulation syndrome]]. Using a [[GnRH agonist]] instead of hCG eliminates most of the risk of ovarian hyperstimulation syndrome, but with a reduced delivery rate if the embryos are transferred fresh.<ref name=Humaidan2011>{{cite journal | vauthors = Humaidan P, Kol S, Papanikolaou EG | title = GnRH agonist for triggering of final oocyte maturation: time for a change of practice? | journal = Human Reproduction Update | volume = 17 | issue = 4 | pages = 510–524 | year = 2011 | pmid = 21450755 | doi = 10.1093/humupd/dmr008 | collaboration = Copenhagen GnRH Agonist Triggering Workshop Group | doi-access = free }}</ref>  For this reason, many centers will freeze all oocytes or embryos following agonist trigger.