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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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


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]
Farlex Partner Medical Dictionary © Farlex 2012


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
The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.


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.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.


1. One of two children born at one birth.
2. Double; growing in pairs.
[A.S. getwin, double]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012


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
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
Medical Dictionary, © 2009 Farlex and Partners


1. One of two children born at a single birth. May be monozygotic or dizygotic.
2. Double; growing in pairs.
[A.S. getwin, double]
Medical Dictionary for the Dental Professions © Farlex 2012

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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