O que (quem) é twin sister - definição
![[[Chang and Eng Bunker]], born in Siam (now [[Thailand]]) in 1811, were the origin of the term "Siamese twins".](https://commons.wikimedia.org/wiki/Special:FilePath/Chang-eng-bunker-PD.png?width=200 "[[Chang and Eng Bunker]], born in Siam (now [[Thailand]]) in 1811, were the origin of the term \"Siamese twins\".")
![Hereford]] breed in [[Miles City, Montana]]](https://commons.wikimedia.org/wiki/Special:FilePath/Hereford twins.jpg?width=200 "Hereford]] breed in [[Miles City, Montana]]")
![[[Ibeyi]] duo Naomi (left) and Lisa-Kaindé Diaz are dizygotic (fraternal) twins.](https://commons.wikimedia.org/wiki/Special:FilePath/Ibeyi by Maya Dagnino 2.jpg?width=200 "[[Ibeyi]] duo Naomi (left) and Lisa-Kaindé Diaz are dizygotic (fraternal) twins.")
![Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same [[placenta]] but have separate [[amniotic sac]]s. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.](https://commons.wikimedia.org/wiki/Special:FilePath/Identical-fraternal-sperm-egg.svg?width=200 "Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same [[placenta]] but have separate [[amniotic sac]]s. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Fraternal twins each have their own placenta and own amniotic sac.")
![A pair of female [[ere ibeji]] twin figures (early 20th-century) in the permanent collection of [[The Children's Museum of Indianapolis]]. The [[Yoruba people]] have the highest dizygotic twinning rate in the world.](https://commons.wikimedia.org/wiki/Special:FilePath/The Childrens Museum of Indianapolis - Female ere ibeji twin figure pair.jpg?width=200 "A pair of female [[ere ibeji]] twin figures (early 20th-century) in the permanent collection of [[The Children's Museum of Indianapolis]]. The [[Yoruba people]] have the highest dizygotic twinning rate in the world.")
![Dichorionic-diamniotic twins at 8 weeks and 5 days since [[co-incubation]] as part of IVF. The twin at left in the image is shown in the [[sagittal plane]] with the head pointing towards upper left. The twin at right in the image is shown in the [[coronal plane]] with the head pointing rightwards.](https://commons.wikimedia.org/wiki/Special:FilePath/Dichorionic twins with heart activity at 8w5d since co-incubation.gif?width=200 "Dichorionic-diamniotic twins at 8 weeks and 5 days since [[co-incubation]] as part of IVF. The twin at left in the image is shown in the [[sagittal plane]] with the head pointing towards upper left. The twin at right in the image is shown in the [[coronal plane]] with the head pointing rightwards.")
![gestational age]] of 15 weeks. There is no sign of any membrane between the fetuses. A [[coronal plane]] is shown of the twin at left, and a [[sagittal plane]] of parts of the upper thorax and head is shown of the twin at right.](https://commons.wikimedia.org/wiki/Special:FilePath/Monoamniotic twins at 15 weeks.jpg?width=200 "gestational age]] of 15 weeks. There is no sign of any membrane between the fetuses. A [[coronal plane]] is shown of the twin at left, and a [[sagittal plane]] of parts of the upper thorax and head is shown of the twin at right.")
Wikipédia
Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In rare cases twins can have the same mother and different fathers (heteropaternal superfecundation).
In contrast, a fetus that develops alone in the womb (the much more common case, in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple. Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgängers.
