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The presence of a "T-sign" at the inter-twin membrane-placental junction is indicative of monochorionic-diamniotic twins (that is, the junction between the inter-twin membrane and the external rim forms a right angle), whereas dichorionic twins present with a "lambda (λ) sign" (that is, the chorion forms a wedge-shaped protrusion into the ...
Monochorionic twins generally have two amniotic sacs (called Monochorionic–Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins, [47] and in 0.3% of all pregnancies. [49] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. [46]
In 1982, twins who were born with two different skin colors were discovered to be conceived as a result of heteropaternal superfecundation. [7] [5] In 1995, a young woman gave birth to diamniotic monochorionic twins, who were originally assumed to be monozygotic twins until a paternity suit led to a DNA test. This led to the discovery that the ...
Monochorionic twins are twins that share the same placenta. This occurs in 0.3% of all pregnancies, [3] and in 75% of monozygotic (identical) twins, when the split takes place on or after the third day after fertilization. [4] The remaining 25% of monozygous twins become dichorionic diamniotic. [4]
One of the twins (Polydeuces, Heracles) is the illegitimate son of the god Zeus; his brother is the son of their mother's mortal husband. A similar pair of twin sisters are Helen (of Troy) and Clytemnestra (who are also sisters of Castor and Polydeuces). The theme occurs in other mythologies as well, and is called superfecundation.
Twins are two offspring produced by the same pregnancy. Twins can either be monozygotic ("identical"), meaning that they develop from one zygote that splits and forms two embryos , or dizygotic ("fraternal"), meaning that they develop from two eggs, each fertilized by separate sperm cells.
Identical twins have identical genomes in the immediate aftermath of twinning. About two-thirds of monozygotic twins share the same placenta, arising by cleavage before the fourth day of development; the other third have separate placentas because cleavage has taken place after the fourth day after choriogenesis has begun.
The other twin is usually normal in appearance. The normal twin, called the pump twin, drives blood through both fetuses. It is called "reversed arterial perfusion" because in the acardiac twin the blood flows in a reversed direction. [4] [5] TRAP sequence occurs in 1% of monochorionic twin pregnancies and 1 in 35,000 pregnancies overall. [6]