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Related to similar twins: monozygotic


one of two offspring produced in the same pregnancy; twins occur about once in every 86 births.

Dizygotic or fraternal twins develop from two separate ova fertilized at the same time. They may be of the same sex or of opposite sexes, and are no more similar than any other two children of the same parents. Called also binovular, dichorial, dissimilar, and unlike twins.

Monozygotic or identical twins develop from a single ovum that divides after fertilization. (See illustration at monozygotic.) Because they share the same set of chromosomes, they are always of the same sex, and are remarkably similar in hair color, finger and palm prints, teeth, and other respects. Monozygotic twins have exactly the same blood type and can accept tissue or organ transplants from each other. Called also enzygotic, monochorial, mono-ovular, similar, or true twins.

Approximately one third of all twins are identical and two thirds are fraternal. It is not clearly understood exactly what causes a single ovum to divide shortly after conception and thereby produce identical twins, although it seems to be a chance occurrence. The reasons for the production and fertilization of two separate ova that result in fraternal twins are not well understood either, but it is thought that a tendency toward fraternal twins runs in families and is transmitted through the genes of the mother. Women are more likely to have fraternal twins in their later childbearing years, between the ages of 30 and 38 years, than earlier. Older age in the father also seems to be a factor with fraternal twins.
conjoined t's Siamese twins.
dizygotic t's (fraternal t's) twins that develop from two separate ova fertilized at the same time; see twin.
identical t's twins that develop from a single ovum; called also monovular or monozygotic twins. See twin.
impacted t's twins so situated during delivery that pressure of one against the other produces simultaneous engagement of both.
mono-ovular t's (monovular t's) (monozygotic t's) identical twins.
Siamese t's monozygotic twins whose bodies are joined; see also siamese twins.
unequal t's conjoined twins (Siamese twins) of which one is incompletely developed.


1. One of two children or animals born at one birth.
2. Double; growing in pairs. May be monozygotic or dizygotic.
[A.S. getwin, double]


(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.
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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.


One of two offspring born at the same birth.
Being two or one of two offspring born at the same birth: twin sisters.
v. twinned, twinning, twins


Etymology: AS, twinn, double
either of two offspring born of the same pregnancy and developed from either a single ovum or from two ova that were released from the ovary simultaneously and fertilized at the same time. The incidence of twin births is approximately 1 in 80 pregnancies. Kinds of twins include conjoined twins, dizygotic twins, interlocked twins, monozygotic twins, Siamese twins, and unequal twins. See also Hellin's law.


One of 2 gestational products that develop during a single intrauterine gestational period. See Dizygotic twin, Monozygous twin, Higher multiples, Partial twin, Siamese twin, Vanishing twin. Cf Siamese twins.


1. One of two children born at one birth.
2. Double; growing in pairs.
[A.S. getwin, double]


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TWINS: A. Monozygotic twins with one placenta, one chorion, and two amnions. B. Dizygotic twins with two placentas, two chorions, and two amnions
Enlarge picture
TWINS: A. Monozygotic twins with one placenta, one chorion, and two amnions. B. Dizygotic twins with two placentas, two chorions, and two amnions
One of two infants born sharing some common anatomical parts. See: illustration; fetus papyraceus; Hellin's law


Per 1000 live births, incidence rates for American whites are 1:88; for American blacks, 1:70. Generally, the rates are higher in blacks and East Indians and lower in Northern Europeans.

Research on Twins

Identical and fraternal twins provide a unique resource for investigating the origin and natural history of various diseases and discovering the different rates of environmental and hereditary factors in causing physical and mental disorders. Esp. important are studies that follow the course of identical twins separated shortly after birth and who then grew up in different social, economic, educational, and environmental conditions. In other research, the second-born twin was found to be at increased risk for an unfavorable outcome (e.g., need for intubation and resuscitation, lower 5-min Apgar score), even when delivered by cesarean section.

biovular twins

Dizygotic twins.

conjoined twins

Twins that are united. In some cases, the individuals are joined in a small area and are capable of activity, but the extent of union may be so great that survival is impossible.

dizygotic twins

Twins from two separate, fertilized ova. Synonym: biovular twins; fraternal twins

enzygotic twins

Monozygotic twins.

fraternal twins

Dizygotic twins.

growth discordant twins

The unequal growth of twins while in utero. The smaller twin is at greater risk of having congenital anomalies than is the normal birth-weight twin.
Synonym: unequal twins

identical twins

Monozygotic twins.

impacted twins

Twins so entwined in utero as to prevent normal delivery.

interlocked twins

Twins in which the neck of one becomes interlocked with the head of the other, making vaginal delivery impossible.

monozygotic twins

Twins that develop from a single fertilized ovum. Monozygotic twins have the same genetic makeup and, consequently, are of the same gender and strikingly resemble each other physically, physiologically, and mentally. They develop within a common chorionic sac and have a common placenta. Each usually develops its own amnion and umbilical cord. Such twins may result from development of two inner cell masses within a blastocyst, development of two embryonic axes on a single blastoderm, or the division of a single embryonic axis into two centers.
Synonym: enzygotic twins; identical twins; true twins; uniovular twins

parasitic twin

The smaller of a pair of conjoined twins, when there is a marked disparity in size.

Siamese twins

A culturally insensitive term for conjoined twins.
See: conjoined twin

true twins

Monozygotic twins.

unequal twins

Growth discordant twins.

uniovular twins

Monozygotic twins.

vanishing twin

Fetal resorption in multiple gestation.
See: gestation, multiple


one of two children born from one pregnancy
  • conjoined twins monozygotic twins that underwent partial separation as a zygote forming two non-separated fetuses (formerly termed Siamese twins); the degree of fetal conjunction and compatibility with normal life is highly variable

  • dizygotic twins; fraternal twins two siblings born from the same pregnancy; the mother formed two eggs, both of which were fertilized

  • monozygotic twins; identical twins twins formed from a single fertilized egg that underwent complete separation as a zygote, forming two genetically identical fetuses that continue to develop as separate individuals


1. One of two children born at a single birth. May be monozygotic or dizygotic.
2. Double; growing in pairs.
[A.S. getwin, double]


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

Patient discussion about twin

Q. anything a mother should know about the last period of twin pregnancy? she is in her 7th month and starting to be real heavy... her legs hurt like crazy. anything to help her with?

A. thanks for all the information! I hope everything will go on well...

Q. Is there a bigger risk of autism for the newborn in twin pregnancy?

A. thanks Dominicus!
you're great...

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