true twins

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Related to true twins: fraternal twins

true twins


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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
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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
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


two offspring produced in the same pregnancy. See also dizygotic and monozygotic twins (below).

conjoined t's
fused, symmetrical twins formed by incomplete division of one embryo into two components. See also siamese twins.
discordant t's
a particular trait is present in only one of the twins.
dissimilar twins
dizygotic twins (below).
dizygotic t's
those that 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 offspring of the same parents. Called also fraternal, binovular, dichorial, dissimilar and unlike twins.
fraternal t's
dizygotic twins.
free symmetrical monozygotic t's
see identical twins (below).
identical t's
derived initially from a single zygote; placentas may be shared or separate.
impacted t's
twins so situated during delivery that pressure of one against the other produces simultaneous engagement in the birth canal of both.
induced twins
produced experimentally in ruminants using PMSG or FSH, or by immunization against androstenedione.
monovular twins
see monozygotic twins (below).
monozygotic t's
those which develop from a single ovum that divides after fertilization. Because they share the same set of chromosomes, they are always of the same sex, and are remarkably similar in haircoat color and pattern, teeth, and other respects. Monozygotic twins have exactly the same blood type and can accept tissue or organ transplants from each other. May be free and symmetrical, free and asymmetrical, cojoined and symmetrical or cojoined and asymmetrical. Called also identical, enzygotic, monochorial, mono-ovular, similar, or true twins.
Siamese t's
cojoined twins. See also siamese twins.
similar twins
see monozygotic twins (above).
symmetrical t's
twins of equal size, age and stage of development.
true twins
see monozygotic twins (above).
unequal t's
twins of which one is incompletely developed, called also asymmetric twins.
unlike twins
see dizygotic twins (above).
References in periodicals archive ?
X, we know that such u, v are true twins and so are at distance 2 in the tree-model.
By the previous claims, all the vertices in Z are true twins.
1] and, since there are no true twins in G, T1 is empty.