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Two offspring from a single pregnancy. Twins may be derived from a single fertilized egg that separates into two after the first division. Such monozygotic twins are genetically identical. Alternatively, twins may be derived from two eggs fertilized at the same time. Such twins are dizygotic and non-identical and in about half the cases are of different sex. The rate of monozygotic twinning in humans is fairly stable at 0.35 per cent, while the rate of dizygotic twinning varies more widely around an average of 1 per cent of pregnancies.


two offspring born at the same time of the same mother. See DIZYGOTIC TWINS, MONOZYGOTIC TWINS.


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).

Patient discussion about twins

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...

More discussions about twins