Step I First Aid Embryo

[DireWolf]

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In the 2003 First Aid on p. 82 it says that monozygotic twins have 1 placenta, 1 chorion, and 2 amniotic sacs. Then it says that monozygotic twins develop 2 placentas (separate/fused), 2 chorions, and 2 amniotic sacs. Which is it supposed to be?

 

[DarksideAllstar]

I thought that this was determined by the day at which they split, but its been so long since Embryo that I cant remember.

 

[Kalel]

My peripheral read on this subject suggests that monozygotic twins can have either of those scenarios depending on when the splitting of the placenta occurs. 70% have monochorionic, diamniotic placentas, and the other 30% have dichorionic, diamniotic placentas.

http://www.emedicine.com/ped/topic2599.htm