Editing Twin (section)

==Statistics==
The human twin birth rate in the United States rose 76% from 1980 through 2009, from 9.4 to 16.7 twin sets (18.8 to 33.3 twins) per 1,000 births.<ref>Martin, Joyce A.; Hamilton, Brady E.; Osterman, Michelle J.K. "Three Decades of Twin Births in the United States, 1980–2009" [https://www.cdc.gov/nchs/data/databriefs/db80.pdf] {{Webarchive|url=https://web.archive.org/web/20171218094748/https://www.cdc.gov/nchs/data/databriefs/db80.pdf|date=2017-12-18}}, National Center for Health Statistics Data Brief, No. 80, January 2012</ref> The [[Yoruba people]] have the highest rate of twinning in the world, at 45–50 twin sets (90–100 twins) per 1,000 live births,<ref>{{cite web |last=Zach |first=Terence |author2=Arun K Pramanik |author3=Susannah P Ford |date=2007-10-02 |title=Multiple Births |url=https://www.emedicine.com/PED/topic2599.htm |url-access=registration |access-date=2008-09-29 |website=[[WebMD]] |archive-date=2008-09-25 |archive-url=https://web.archive.org/web/20080925000450/http://www.emedicine.com/ped/TOPIC2599.HTM |url-status=live}}</ref><ref>{{cite news |url=https://www.iol.co.za/news/africa/genetics-or-yams-in-the-land-of-twins-378435|title=Genetics or yams in the Land of Twins?|newspaper=Independent Online |date=2007-11-12|access-date=2008-09-29|archive-date=2017-02-23|archive-url=https://web.archive.org/web/20170223211901/http://www.iol.co.za/news/africa/genetics-or-yams-in-the-land-of-twins-378435|url-status=live}}</ref><ref>{{Cite news|url=https://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml|title=The Land of Twins|publisher=[[BBC World Service]]|date=2001-06-07|access-date=2008-09-29|archive-date=2008-12-15|archive-url=https://web.archive.org/web/20081215173510/http://www.bbc.co.uk/worldservice/people/highlights/010607_twins.shtml|url-status=live}}</ref> possibly because of high consumption of a specific type of [[yam (vegetable)|yam]] containing a natural [[phytoestrogen]] which may stimulate the [[ovary|ovaries]] to release an egg from each side.<ref>{{Cite book|title=Multiple Pregnancy: Epidemiology, Gestation & Perinatal Outcome|editor1=Louis G. Keith |editor2=Emile Papierik |editor3=Donald M. Keith |editor4=Barbara Luke |author1 =O. Bomsel-Helmreich|author2=W. Al Mufti |isbn=978-1-85070-666-3|publisher=Taylor and Francis |chapter=The mechanism of monozygosity and double ovulation|year=1995|page=34}}</ref><ref>{{cite web |year=1999 |title=What's in a yam? Clues to fertility, a student discovers |url=https://medicine.yale.edu/news/yale-medicine-magazine/article/whats-in-a-yam-clues-to-fertility-a/ |access-date=2022-04-07 |website=Yale Medicine Magazine |archive-date=2022-05-07 |archive-url=https://web.archive.org/web/20220507064248/https://medicine.yale.edu/news/yale-medicine-magazine/article/whats-in-a-yam-clues-to-fertility-a/ |url-status=live}}</ref> In [[Central Africa]], there are 18–30 twin sets (or 36–60 twins) per 1,000 live births.<ref name="Smits">{{Cite journal
|title=Twinning across the Developing World|journal=PLOS ONE |year=2011 |volume=6 |issue=9 |pages=e25239 |last=Smits |first=Jeroen |author2=Christiaan Monden |doi=10.1371/journal.pone.0025239|editor1-last=Newell |editor1-first=Marie-Louise |pmid=21980404 |pmc=3182188 |bibcode=2011PLoSO...625239S|author2-link=Christiaan Monden|doi-access=free}}</ref> In [[South America]], [[South Asia]], and [[Southeast Asia]], the lowest rates are found; only 6 to 9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9 to 16 twin sets per 1,000 live births.<ref name="Smits" />

Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting on average 37 weeks, three weeks less than full term.<ref>{{Cite journal |last=Elliott |first=JP |date=December 2008 |title=Preterm labor in twins and high-order multiples |journal=Clinics in Perinatology |volume=34 |issue=4 |pages=599–609 |doi=10.1016/j.clp.2007.10.004 |pmid=18063108 |quote=Unlike singleton gestation where identification of patients at risk for PTL is often difficult, every multiple gestation is at risk for PTL, so all patients can be managed as being at risk.}}</ref> Women who have a family history of fraternal twins have a higher chance of producing fraternal twins themselves, as there is a genetically linked tendency to hyper-[[Ovulation|ovulate]]. There is no known genetic link for identical twinning.<ref>{{cite web |title=Probability of having twins hereditary? |url=https://goaskalice.columbia.edu/answered-questions/probability-having-twins-hereditary |access-date=2022-04-07 |work=Go Ask Alice! |publisher=Columbia University |archive-date=2022-09-25 |archive-url=https://web.archive.org/web/20220925203114/https://goaskalice.columbia.edu/answered-questions/probability-having-twins-hereditary/ |url-status=live}}</ref> Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births.<ref>{{cite web |last=Fitch |first=Karen |date=26 January 2005 |title=My father is an identical twin and my husband's grandfather is a fraternal twin. Does this increase our changes of having twins, or is the genetic trait only on the mother's side and only for fraternal twins? |url=https://www.thetech.org/ask-a-geneticist/ask86 |access-date=2022-04-07 |work=Ask a Geneticist |publisher=The Tech Interactive |archive-date=2022-06-24 |archive-url=https://web.archive.org/web/20220624225411/https://www.thetech.org/ask-a-geneticist/ask86 |url-status=live}}</ref> Some women intentionally turn to fertility drugs in order to conceive twins.<ref>{{cite news |title=China: Drug bid to beat child ban |url=https://www.chinadaily.com.cn/english/doc/2006-02/14/content_520025.htm |access-date=11 November 2008 |date=14 February 2006 |agency=Associated Press |work=China Daily |url-status=live |archive-date=6 December 2008 |archive-url=https://web.archive.org/web/20081206001831/http://www.chinadaily.com.cn/english/doc/2006-02/14/content_520025.htm}}</ref><ref>{{cite web |title=Chinese Women Try to Bypass One-Child Policy With Pills for Twins |publisher=ABC News |date=August 3, 2011 |url=https://abcnews.go.com/Health/chinese-women-fertility-drugs-bypass-child-policy/story?id=14219173}}</ref>