One of two children produced in the same pregnancy and born during the same birth process. Twins can develop from one ovum (egg) or from two ova (eggs): {{}}Twins from one ovum: Twins that develop from a single ovum are called monozygotic. They are said to identical twins and have identical genomes. Twins from two ova: Twins that develop from two ova that are fertilized at the same time are called dizygotic. They are nonidentical twins and have different genomes. The term "fraternal" is often applied to nonidentical twins but it may be misleading. Nonidentical twins can be of the same sex (two brothers or two sisters) or of different sex (brother and sister). The better term is "nonidentical."
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1. One of two children born at one birth. 2. Double; growing in pairs. [A.S. getwin, double]
- allantoidoangiopagous twins unequal monochorial twins with fusion of their allantoic vessels within the placenta; the lesser t. is essentially a parasite on the placental circulation of the larger t..
- conjoined twins monozygotic twins with varying extent of union and different degrees of residual duplication. The various types of union are named by the use of a prefix designating the region that is united and adding the suffix -pagus, meaning joined ( e.g., craniopagus, thoracopagus); the various types of residual duplication are named by designating the parts duplicated and adding the suffix -didymus, or -dymus, meaning t. ( e.g., cephalodidymus, cephalodymus).
- conjoined asymmetric twins SYN: conjoined unequal twins.
- conjoined equal twins conjoined twins in which both members are approximately of the same size, and nearly normal except for the areas of union. SYN: conjoined symmetric twins.
- conjoined symmetric twins SYN: conjoined equal twins.
- conjoined unequal twins conjoined twins in which one member is nearly normal (host or autosite) and the other (parasite) is small, incomplete, and dependent for its nutrition upon the more nearly normal member. SYN: conjoined asymmetric twins.
- dichorial twins SYN: dizygotic twins.
- diovular twins SYN: dizygotic twins.
- dizygotic twins twins derived from two separate zygotes. SYN: dichorial twins, diovular twins, fraternal twins, heterologous twins.
- enzygotic twins SYN: monozygotic twins.
- fraternal twins SYN: dizygotic twins.
- heterologous twins SYN: dizygotic twins.
- identical twins SYN: monozygotic twins.
- incomplete conjoined twins conjoined twins, the two components of which equal one another but are less than entire individuals.
- locked twins a form of malpresentation in which a breech t. and a vertex t. become locked at the chin during labor and attempted delivery.
- monoamniotic twins twins within a common amnion; such t.'s are monovular in origin and may be conjoined.
- monochorial twins SYN: monozygotic twins.
- monovular twins SYN: monozygotic twins.
- monozygotic twins twins resulting from a single fertilized ovum that at an early stage of development becomes separated into independently growing cell aggregations giving rise to two individuals of the same sex and identical genetic constitution. SYN: enzygotic twins, identical twins, monochorial twins, monovular twins, uniovular twins.
- parasitic t. the smaller of unequal conjoined twins.
- polyzygotic twins twins resulting from fertilization of more than two ova discharged in a single ovulating cycle.
- Siamese twins originally, a much publicized conjoined pair of twins (xiphopagus) from Siam in the 19th century; this term has since come into general lay usage for any type of conjoined twins, but is incorrect.
- uniovular twins SYN: monozygotic twins.

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twin 'twin adj born with one other or as a pair at one birth <a \twin brother> <\twin girls>
twin n
1) either of two offspring produced at a birth
2) twins pl a group of two offspring born at one birth
twin·ship -.ship n

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(twin) one of two offspring produced in the same pregnancy; see monozygotic t's and dizygotic t's.

Twins. (A), The most common type of monozygotic twinning, with division of the inner cell mass of the blastocyst resulting in separate amnions but a single chorion and placenta; (B), a rare form of monozygotic twinning, with complete division of the embryonic disc resulting in two embryos in a single amniotic sac with a single placenta and chorionic sac; (C), monozygotic twinning with division occurring between the two-cell and morula stages to produce identical blastocysts, resulting in separate amniotic and chorionic sacs and either separate (shown) or fused placentas; (D,E), dizygotic twinning, with (D) or without (E) fusion of the placenta and chorion.

Medical dictionary. 2011.


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