Interuterine Appearance of Twins:
Identical twins can have their own, or share both the outer sac (chorion) and the inner sac (amnion):
1 chorion, 2 amnions, 1 placenta (most common) roughly 65-70%
2 chorions, 2 amnions, fused placentas
2 chorions, 2 amnions, separate placentas (about 25-30%, this includes fused placentas)
1 chorion, 1 amnion, 1 placenta (rare) 1-5% These are termed monoamniotic/monochorionic twins.
If identical twins split early after fertilization, then each twin will have it's own chorion and amnion. If the egg splits somewhere between 4-8 days after, each will have it's own amnion, but share a chorion, about 2/3 of twins are like this. If they split after the 8th day, they will be monochorionic/monoamniotic, and anytime after the 12th day, they will be conjoined. Some researchers believe that monochorionic/amniotic twins were very close to being conjoined.
Fraternal twins always have their own chorions and amnions, as well as placentas, but the placentas may become fused and look like one placenta, so it may not always be possible to tell if your twins are identical or fraternal from the placentas. Fraternal twins can't share a chorion or amnion.
It is difficult to tell if twins are identical or fraternal from an ultrasound unless they are monochorionic/monoamniotic or there is only one chorion clearly visible. One chorion would mean the twins are identical.
The only sure way to know if your twins are identical or fraternal (barring them being of different sexes) is a DNA test. I hope this information helps out! I am a mother of twins that are now 9years old, they had one placenta but seperate amnion and chorion, my OB said they were fraternal based on examination of the placenta. My boys look identical and I can't even tell them apart at times, and neither can their teachers or friends.
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