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Placenta

Placental findings in specific newborn/fetal or maternal conditions

Twins

Reviewers: Mandolin Ziadie, M.D. (see Reviewers page)
Revised: 15 November 2011, last major update November 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.

General
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● 1 in 80 births in US, 1/3 are monozygous
● Dichorionic, diamnionic: may be mono or dizygotic; lack vascular anastomoses because in cases of a single disc, the septum / dividing membrane is an area of fusion rather than a true single disc
● Monochorionic: are monozygotic; may be diamnionic or monoamnionic, depending on timing of split
● Monochorionic have higher mortality due to premature labor; monoamniotic have 33% perinatal mortality due to cord complications (twist, knot)

Gross description
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● Didi discs may be entirely separate or fused
● When fused, fetal vessels do not cross area of fusion
● The dividing membranes are usually thick / opaque with grossly visible blood vessels and a distinct ridge / septum where they connect to the disc

● Mono discs may have major vascular anastomoses between the twins with arteries superficial to veins
● It is important to document the type, size and location of anastamoses, using injection studies to show the flow of blood
● Monochorionic diamniotic (MoDi): the dividing membranes are usually thin and translucent without a thick ridge at the point of attachment
● Monochorionic monoamniotic (MoMo): no dividing membranes; conjoined twins fall into this category

● Abnormalities in cord insertion (e.g. velamentous cords) are more common in twins

Micro description
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● The t-zone is the region of the disc where the dividing (septal) membranes attach to the chorionic plate
● Sampling this region or making a membrane roll of the dividing membranes will provide information about the type of twinning
● DiDi (dichorionic diamniotic): "hamburger" consisting of a layer of chorion between two amniotic membranes
● MoDi (monochorionic-diamniotic): "nothing" (translucent septum) in the middle of the two amnionic layers

Micro images
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Left: DiDi, right: MoDi

Diagrams
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Twinning

End of Placenta > Placental findings in specific newborn/fetal or maternal conditions > Twins

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