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fraternals in same sac?

post #1 of 13
Thread Starter 
Is it possible for fraternal twins to share a sac? And then have this happen?


Another poster on another thread said, "fraternal twins often share a sac." Is this true?
post #2 of 13

fraternal twins

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.
post #3 of 13
Okay... either the news story is a hoax or fraternals can share a sac. Or one of those babies has incomplete genetalia and they need to run some chromosome tests to figure out what's up.
post #4 of 13
Okay Darylll... I suspect CNN screwed up. Here is another report on the same story.
ROME (AP) - Four-month-old twin girls joined at the temple were successfully separated after 12 hours of surgery at a Rome hospital, a hospital official said.
Same dates, same story, born in Greece, surgery in Italy.
post #5 of 13
Thread Starter 
post #6 of 13
From what I understand, all siamese twins are identical, the joining of the twins being a result of incomplete seperation of the egg. Is this correct?
post #7 of 13
DalaiMama... I believe you are correct. That is why Darylll and I were REALLY thrown by a CNN report of boy/gorl conjoined twins!
post #8 of 13
Thanks Kama!
post #9 of 13
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?
post #10 of 13

Your twins could be either...

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.)


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.


mama to a 3.5 yr old boy and twins? (expected in April 2004)
post #11 of 13
Kathryne - thank you so much for that explanation. I still don't know for sure but at least will be able to ask my doctor some more questions when we see her.
post #12 of 13
Thread Starter 
CherylE. Congrats on your twin pregnancy!

Asking your doctor is one way to get info. If you would like to inform yourself more fully, twin moms recommend these two books:

for pregnancy:

When You're Expecting Twins, Triplets, or Quads


for when the babies come (the best bfing info out there for multiples):

Mothering Multiples

post #13 of 13

It's possible for MZ twins to be different genders

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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