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. 
Siamese twins
'Siamese' 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.
Siamese twins
[sī′əmēz]
Etymology: Chang and Eng, conjoined twins born in Siam (now Thailand) in 1811
conjoined, equally developed twin fetuses produced from the same ovum. The severity of the condition ranges from superficial fusion, such as of the umbilical vessels, to that in which the heads or complete torsos are united and several internal organs are shared. With modern surgical techniques, most Siamese twins can be successfully separated. See also conjoined twins.
Siamese twins
identical (monozygotic) twins joined together at birth. The connection may be slight or extensive. It involves skin and usually muscles or cartilage of a limited region, such as the head, chest, hip or buttock. The twins may share a single organ, such as an intestine, or occasionally may have parts of the spine in common. A rare congenital malformation in animals. Called also conjoined twins.
'Siamese' twins
Conjoined equal twins A joined gestational product due to a failure in division of the yolk sac or due to delayed monovular separation; STs occur in ±1:200,000 term deliveries; most are joined at the
chest–thoracopagus Prognosis Depends on adequacy of surgical separation
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