Editing Twin (section)

===Predisposing factors===
The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
* She is of [[West Africa]]n descent (especially [[Yoruba people|Yoruba]])
* She is between the age of 30 and 40 years
* She is greater than average height and weight
* She has had several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. In the United States it has been estimated that by 2011 36% of twin births resulted from conception by [[assisted reproductive technology]].<ref>{{Cite journal |last1=Kulkarni |first1=A. D. |last2=Jamieson |first2=D. J. |last3=Jones |first3=H. W. Jr. |last4=Kissin |first4=D. M. |last5=Gallo |first5=M. F. |last6=MacAluso |first6=M. |last7=Adashi |first7=E. Y. |doi=10.1056/NEJMoa1301467 |title=Fertility Treatments and Multiple Births in the United States |journal=New England Journal of Medicine |volume=369 |issue=23 |pages=2218–2225 |year=2013 |pmid=24304051| doi-access=free}}</ref>

The risk of twin birth can vary depending on what types of fertility treatments are used. With [[in vitro fertilisation]] (IVF), this is primarily due to the insertion of multiple embryos into the uterus. [[Controlled ovarian hyperstimulation|Ovarian hyperstimulation]] without IVF has a very high risk of multiple birth. Reversal of [[anovulation]] with [[clomifene]] (trade names including ''Clomid'') has a relatively less but yet significant risk of multiple pregnancy.