Types of Twins and Twinning (Chorion and Amnion) - USMLE Forums
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Old 11-01-2009
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ObGyn Types of Twins and Twinning (Chorion and Amnion)

The types of twins whether they are diamnionic dichorionic or diamnionic monochorionic or monoamnionic monochorionic are usually asked in USMLE step 1 exam.

It all depends at the time of separation check this illustration

Click to enlarge
Types of Twins and Twinning (Chorion and Amnion)-twinning-time.jpg

Note in the illustration above that there's no such thing as monoamnionic dichorionic because the chorion is developed before the amnion.

Monochorionic placentae are found only in identical twins while dichorionic twins can be seen in both identical (monozygotic) and fraternal (dizygotic) twins.

Early separation at the two-cell stage usually result in two chorions and two amnions while late separation usually result in single amnion and single chorion with the others in between as seen in the illustration above.

Specific Timing of Separation and Resultant Twin Type:
First 72 hours (up to morula stage): Dichorionic Diamnionic (No risk of Twin-Twin transfusion syndrome)
Between 4 and 8 Days (Blastocyst stage): Monochorionic Diamnionic
Between 9 and 12 Days (Splitting of embryonic disk) ---> Monochorionic Monoamnionic
Any separation after that results in Conjoined Twins and is usually lethal

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Old 11-01-2009
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Wonderful post
Thanks Sabio for the great work
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Embryology-, Figures-, ObGyn-

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