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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.
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.
twinOne 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.
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 twinsDizygotic twins.
enzygotic twinsMonozygotic twins.
fraternal twinsDizygotic twins.
growth discordant twins
identical twinsMonozygotic twins.
true twinsMonozygotic twins.
unequal twinsGrowth discordant twins.
uniovular twinsMonozygotic twins.
twinone 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
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?
Q. Is there a bigger risk of autism for the newborn in twin pregnancy?