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Fraternal twins cannot share a sac. The sac is made from certain cells of the newly forming baby. Fraternal twins were always seperate and cannot share a sac nor any tissue. Sometimes it can appear that fraternal twins share a placenta, but it's really that the two placentas grew together and fused. This is why it can be confusing to figure out if twins are fraternal or identical by just looking at the placenta. A genetics test can clear up any confusion. Identical (or monozygotic) twins may share a sac or they may not. It depends on when the separation occured.
 

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So if there is only one placenta - does that mean the twins are identical? I'm asking because I just found out at my 18 week ultrasound this afternoon that we are having twins. The tech said that there was definately only one placenta, but she couldn't tell if they were in the same sack or not for sure - though they might be. So she couldn't say if the twins are identical or fraternal. Anybody know?
 

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If there is definately only one placenta, then they are identical. But two placentas can become fused and then look like only one placenta, especially on u/s. (Fusing can happen in the early early weeks, so it's possible for that to already be the case at 18 weeks.)

If the twins are b/g (boy/girl), then they are definately dizygotic (two eggs). If they are b/b or g/g then sometimes the only way to know is through zygosity testing. (Zygosity testing is an easy process involving swabbing the inside of their mouths with a q-tip-like-thing, and sending it to a lab---they evaluate the genetic material from there and let you know.)

So...

DZ (dizygotic, two eggs, fraternal) twins can be either b/g, b/b, or g/g, and each have their own placentas, their own chorion, and their own amnion.

MZ (monozygotic, one egg, or identical twins) are always the same sex and can have (a) their own placentas (which can become fused), their own chorions, and their own amnions, (b) one placenta, one chorion, and two amnions, or (c) (and most rare) one placenta, one chorion, and one amnion.

What the tech said is confusing because "in the same sac" means different things to different people. The amniotic sac is typically though of as being just one bag of waters, but it really has several layers. Establishing whether or not they share the same chorion and amnion is the important thing.

Congratulations!

Kathryne
mama to a 3.5 yr old boy and twins? (expected in April 2004)
 

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But it's *extremely* unlikely. I read a website on Statistics & Multiple birth, and the webmaster could only find 3 instances of it occurring. Here's what happened: a fertilized egg with XY chromosomes (boy) split incompletely, leaving one embryo with either a Y or an XY and the other with just an X, making the second embryo, by default, a female with Turner's Syndrome. Not that I'm sure that's what happened in the above posted case, because I haven't checked the link, but I thought you might like to know that it is possible. The twins would still be monozygotic (from one egg), but they would not be "identical", so to speak. Strange, but true.
 
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