|
Twins are two offspring produced by the same pregnancy.〔(MedicineNet > Definition of Twin ) Last Editorial Review: 6/19/2000〕 Twins can either be ''monozygotic'' ("identical"), meaning that they can develop from just one zygote that will then split and form two embryos, or ''dizygotic'' ("fraternal"), meaning that they can develop from two different eggs, each are fertilized by separate sperm cells.〔Michael R. Cummings "Human Heredity Principles and issues" pg 104〕 In contrast, a fetus which develops alone in the womb is called a ''singleton'', and the general term for one offspring of a multiple birth is ''multiple''.〔(【引用サイトリンク】title=Twins, Triplets, Multiple Births: MedlinePlus )〕 ==Statistics== The twin birth rate in the United States rose 76% from 1980 through 2009, from 18.9 to 33.3 per 1,000 births.〔Martin, Joyce A.; Hamilton, Brady E.; Osterman, Michelle J.K. "Three Decades of Twin Births in the United States, 1980–2009" (), National Center for Health Statistics Data Brief, No. 80, January 2012〕 (The first successful 'test tube baby' was born in 1978.) The Yoruba have the highest rate of twinning in the world, at 45–50 twin sets (or 90–100 twins) per 1,000 live births, possibly because of high consumption of a specific type of yam containing a natural phytoestrogen which may stimulate the ovaries to release an egg from each side.〔(【引用サイトリンク】title=What’s in a yam? Clues to fertility, a student discovers )〕 In Central Africa there are 18–30 twin sets (or 36–60 twins) per 1,000 live births. In Latin America, South Asia, and Southeast Asia, the lowest rates are found; only 6–9 twin sets per 1,000 live births. North America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.〔 Multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average. 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-ovulate. There is no known genetic link for identical twinning.〔(【引用サイトリンク】url=http://goaskalice.columbia.edu/probability-having-twins-hereditary )〕 Other factors that increase the odds of having fraternal twins include maternal age, fertility drugs and other fertility treatments, nutrition, and prior births. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Twin」の詳細全文を読む スポンサード リンク
|