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. 
monozygotic twins,
monozygotic twins
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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.
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.
monozygotic twins,
two offspring born of the same pregnancy and developed from a single fertilized ovum that splits into equal halves during an early cleavage phase in embryonic development, giving rise to separate fetuses. Such twins are always of the same sex, have the same genetic constitution, possess identical blood groups, and closely resemble each other in physical, psychologic, and mental characteristics. Monozygotic twins may have single or separate placentas and membranes, depending on the time during development when division occurs. Monozygotic twinning occurs with relatively uniform frequency in all races, is unaffected by heredity, and represents approximately one third of all twin births. Also called enzygotic twins, identical twins, true twins, uniovular twins. Compare dizygotic twins. See also Siamese twins.
monozygotic
pertaining to or derived from a single zygote (fertilized ovum); said of twins.
monozygotic twins
see monozygotic twins.
monozygotic twins
Identical twins Twins resulting from the division of a single fertilized egg, which usually share a common chorion and placenta; usually each has a separate amnion. Cf Fraternal twins.
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