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Chorionicity of Twin Membranes
LifeART (and/or) MediClip image copyright 2008
Wolters Kluwer Health, Inc.- Lippincott Williams & Wilkins. All rights reserved.
The placenta has attached the fetal membranes, which consist of the amnion and the chorion. These two very thin layers are the fetal membranes and contain the baby and the amniotic fluid in a singleton pregnancy. In a multiple gestation you can have various combinations, including:
- Diamniotic/Dichorionic (DiDi)
This means that your babies have their own amnion and their own chorion. They are in separate membranes. This can happen with dizygotic (fraternal) twins as well as monozgotic (identical) twins. The placentas will be separate, but may fuse. When this happens in identical twins, it is determined by when the egg split. The sooner it splits the more likely you are to have separate placentas and membranes.
- Monochorionic/Diamniotic (MoDi)
These are more likely identical or monozygotic twins who share the outer layer, the chorion, but have individual inner sacs, the amnion. There are some dizygotic twins who will have this type of chorionicity.
- Monochorionic/Monoamniotic (MoMo)
This is the most rare, but potentially very dangerous situation where the babies share both the outer and inner sac, known as monoamniotic. The risks here are largely about cord entanglement and twin-to-twin transfusion syndrome (TTTS).
The following pages contain shots of actual twin placentas.