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. 
dizygotic twins,
dizygotic twins
)
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.
dizygotic twins,
two offspring born of the same pregnancy and developed from two ova that were released from the ovary simultaneously and fertilized at the same time. They may be of the same or opposite sex, differ both physically and genetically, and have two separate and distinct placentas and membranes, both amnion and chorion. The frequency of dizygotic twinning varies according to ethnic origin (the highest incidence occurs in African-Americans, the lowest in Asian-Americans, with Caucasians intermediate), maternal age (the highest rate occurs when the mother is 35 to 39 years of age), and heredity (showing an increase in the female genetic line rather than the male, although fathers may transmit the disposition to double ovulation to their daughters). In general the overall ratio is two thirds dizygotic twinning to one third monozygotic. Also called binovular twins, dissimilar twins, false twins, fraternal twins, heterologous twins.
dizygotic twins
Fraternal twins Twins resulting from 2 separate fertilized eggs, liberated simultaneously from the ovaries, which develop in separate or partially fused chorion and placenta, and usually a separate amniotic sac. Cf Monozygotic
twins.
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