Twin

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are more often handled with special procedures than regular births.

References

• {{Journal reference issue | Author=Fraga MF, Ballestar E, Paz MF, Ropero S, Setien F, Ballestar ML, Heine-Suner D, Cigudosa JC, Urioste M, Benitez J, Boix-Chornet M, Sanchez-Aguilera A, Ling C, Carlsson E, Poulsen P, Vaag A, Stephan Z, Spector TD, Wu YZ, Plass C, Esteller M | Title=Epigenetic differences arise during the lifetime of monozygotic twins | Journal=Proceedings of the National Academy of Sciences | Volume=Epub ahead of print | Issue= | Year=2005 | Pages=}} PMID 16009939
• {{Journal reference issue | Author=Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. | Title='Identical' twins with discordant karyotypes | Journal=Prenatal Diagnosis | Volume=19 | Issue=1 |Year=1999 | Pages=72-6}} PMID 10073913
• {{Journal reference issue | Author=Wenk RE, Houtz T, Brooks M, Chiafari FA | Title=How frequent is heteropaternal superfecundation? |Journal=Acta geneticae medicae et gemellologiae | Year=1992 | Volume=41 | Issue=1 | Pages=43-7 }} PMID 1488855
• {{Journal reference issue | Author=Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E | Journal=Fertility and Sterility | Title=Indisputable double paternity in dizygous twins | Year=1997 | Volume=67 | Issue=6 | Pages=1159-61 }} PMID 9176461