Can fraternal twins share a chorionic sac?

I know this seems like a stupid question - but at an early ultrasound, can you know automatically if twins are identical if they share a chorionic/gestational sac? Is that always true? I also understand that sharing a chorionic sac doesn't always mean a shared placenta, that it is possible to have two placentas form for each twin, within the same chorionic sac.

In our case, we know our twins are identical because they are sharing a placenta (we're pretty sure, anyway) - but I wonder if we'd have known this a lot earlier, if sharing a chorionic sac also meant identicals.

Just for a pic - here's a pic of our early ultrasound, showing two babies within the same chorionic sac, rather than two separate ones....

http://i2.photobucket.com/albums/y2/colofisch/us1.jpg

Thanks, and just curious!

Jenny

 

No fraternal twins cannot share a sac. Fratenal twins share nothing in utero.
The chorionic sac is actually part of the placenta, sort of. If forms from the same part of the fertilized egg that the placenta comes from.

 

From what I understand, the only way to tell if they're identical is through a blood test. Sharing a placenta makes it more likely, but doesn't necessarily mean identical. I've been reading up on it because my boys share a placenta, and I'm hoping that they are identical! I could be wrong, but this is what I keep coming across.

 

As far as was explained to us by our dr & two ultrasound techs, fraternal twins only share the overall womb & nothing else (ie: sacs, placenta, fluid ect) Identicals can be in different sacs sometimes, but we were told they always share the placenta.
We asked cause when we went for our 1st ultrasound @ 7 wks & then again @ 13wks, both girls who did the ultrasounds told us our babies were definatly fraternal because they didnt' share anything & the dr. said that was absolutely right.

 

The number of placentas is not always an indicator of identical twins. If the egg splits early enough each twin will have their own placenta, if it splits a few days later they will share one. If the egg has not split by 13-15 days the twins will be conjoined. (just some fun info my u/s tech told me the other day)

Did your doctor look for a dividing membrane?? They look like they are in the same sack from the u/s picture, but sometimes the membrane is just really thin and hard to see. If there is no membrane then your twins are mo/mo twins and they are considered high risk because their cords could get tangled together. I don't want to scare you, but if your twins are sharing a sack your doctor should have said something because it changes the level of care you will need for the pregnancy.

 

QUOTE(Susiepie @ Jul 16 2007, 05:30 PM) [snapback]333384[/snapback]
Did your doctor look for a dividing membrane?? They look like they are in the same sack from the u/s picture, but sometimes the membrane is just really thin and hard to see. If there is no membrane then your twins are mo/mo twins and they are considered high risk because their cords could get tangled together. I don't want to scare you, but if your twins are sharing a sack your doctor should have said something because it changes the level of care you will need for the pregnancy.


Don't worry - nothing much can scare me anymore. They think they found the membrane - once, at the 16 week u/s - which would make them mo/di twins - but still identical I think - because they started out in one chorionic sac - whether they develop their own placentas or not (in our case, maybe not, since they think we're dealing with TTTS - which is also scary, maybe even as scary a mo/mo twins, I don't know!). They haven't found the membrane since though - but I think it's there somewhere... (even if shrink-wrapped around our smaller baby) - so we're alreayd high-risk!

I just feel dumb, because even after the first u/s, I thought they could still be fraternal (I have SO many twins on my side of the family), but, apparently, our twins have nothing to do with it! And, we got a million different answers from our OB's so I thought I'd come here to ask the experts!

Jenny

 

QUOTE(rudy893 @ Jul 16 2007, 04:00 PM) [snapback]333339[/snapback]
As far as was explained to us by our dr & two ultrasound techs, fraternal twins only share the overall womb & nothing else (ie: sacs, placenta, fluid ect) Identicals can be in different sacs sometimes, but we were told they always share the placenta.
We asked cause when we went for our 1st ultrasound @ 7 wks & then again @ 13wks, both girls who did the ultrasounds told us our babies were definatly fraternal because they didnt' share anything & the dr. said that was absolutely right.


This is incorrect. A shared sac or placenta always indicates identical but sharing nothing does NOT equal fraternal. Mine are as identical as identical gets (I have the genetic test to prove it) and they shared nothing but the womb just like a couple of fraternal twins. This is what happens if the egg splits in the first 3 days.

The tricky part is that fused placentas (two that implanted closely together and overlapped as they grew) can look a lot like a shared placenta. There are moms on TS who were told they were having ID twins and later learned they were fraternal as well as people who were told they were having frats and later learned they were identical.

 

So, even before you learn that the twins share a placenta, can you know they're identical from the early ultrasound where they share a chorionic sac? is that definitive?

jenny

 

If they share Anything for sure, then they are identical...the tricky part is that if they are in different sacs and have placentas that are very close, possibly fused looking, then frats might get mistaken for identical...but if they share a sac, then for sure they are identical. The converse is not true...if they share Nothing then they ARE fraternal...that is not always true. Jenny, if your twins share a sac, they are identical, no doubt about it. And from the pic you showed, it sure looked like they share a sac.