twin

twin (twin) one of two offspring produced in the same pregnancy and developed from one oocyte (monozygotic) or from two oocytes (dizygotic) fertilized at the same time.

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.

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allantoidoangiopagous twins  twins united by the umbilical vessels only.

conjoined twins  monozygotic twins whose bodies are joined to a varying extent.

diamniotic twins  twins developing within separate amniotic cavities; they may be monochorionic or dichorionic.

dichorionic twins  twins having distinct chorions, including monozygotic twins separated within 72 hours of fertilization and all dizygotic twins.

dizygotic twins , fraternal twins, heterologous twins twins developed from two separate oocytes fertilized at the same time.

identical twins  monozygotic t's.

impacted twins  twins so situated during delivery that the pressure of one against the other prevents simultaneous engagement of both.

monoamniotic twins  twins developing within a single amniotic cavity; they are always monozygotic and monochorionic.

monochorionic twins  twins developing with a single chorion; they are always monozygotic and may be monoamniotic or diamniotic.

monozygotic twins  two individuals developed from one fertilized oocyte; they have identical genomes.

omphaloangiopagous twins  allantoidoangiopagous t's.

Siamese twins  conjoined t's.

similar twins  monozygotic t's.

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twin (twn)

n.

One of two offspring born at the same birth.

adj.

1. Being two or one of two offspring born at the same birth.

2. Consisting of two identical or similar parts; double.

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twin

one of the two same-age fetuses in the one pregnancy. See also twins, twinning.