can someone explain to me mono vs. di and how they can tell via U/S? - Mothering Forums
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#1 of 6 Old 10-27-2012, 07:11 AM - Thread Starter
 
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I've tried to read up on it, but I find it all confusing. Here is what I think I know (please correct me!):

It can be mono-mono, mono-di, di-di and that has to do with one vs two zygotes and one vs two sacks

Dizygotic are definitely fraternal and monozygotic are definitely identical?
And how can they tell via u/s? Do you need an early u/s to determine it or will they be able to tell me at my 20 week u/s?
Anything else I should know?
I appreciate your knowledge! 


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#2 of 6 Old 10-27-2012, 11:51 AM
 
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Yes, dizygotic are fraternal and monozygotic are identical.  But that is different than when we're using the terms di/di or mono/di, etc.  The zygote is referring to the egg - di = there were two eggs, mono = there was one that split.  The term di/di is short for dichorionic, diamniotic.  It's referring to the placenta (chorion) and the sack (amnion): di/di = two sacks, two placentas.  

 

All fraternal twins are di/di (although sometimes placentas fuse and look like they're mono/di).  20-30% of identicals are also di/di.  A mono/mono is definitely identical, and a mono/di could be either, although I think a skilled tech can tell if the placentas are really single or two that fused, and if they're really single it's definitely identical.    

 

Why do we bother with knowing di/di, mono/di or mono/mono?  There are different risks associated with each of the three options.  di/di twins are in the lowest risk category.  mono/di twins are at risk for TTTS (twin-twin transfusion syndrome), and mono-mono twins are very high risk, since they share one sack they can get tangled in each other's umbilical cords, and they're also at risk for TTTS.  di/di twins can still have discordant growth - it's not as serious as TTTS, but that's why they want to monitor your twin's growth with ultrasound regularly.  

 

Yes, they'll be able to tell you at your 20 week u/s.  I'm not sure if it's easier to tell if the placentas are fused or single earlier on or not.

 

Good luck at your u/s!  keep us posted.


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#3 of 6 Old 10-27-2012, 12:11 PM - Thread Starter
 
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Thank you so much. That was incredibly helpful!


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#4 of 6 Old 10-27-2012, 02:48 PM
 
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Prior to 14w is said to be the best time to.determine chorionicity. If they can't tell.for sure, they should treat the rest of the pregnancy as if they were mono/di.
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#5 of 6 Old 10-27-2012, 09:41 PM
 
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there is a chance that a later U/S like a 20 week one will not be able to see the membrane between the babies well any more and that things are too crowded to see the T or the "Lamba" where the innr and out sacks relate to eachother, the tell tell signs of separation that is the difference between di/di and Mo/di, so that is the reason that is usually better to find out earlier.   and because finding out earlier lets you understand and be educated on the issues of each type.


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#6 of 6 Old 10-28-2012, 04:42 PM
 
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This is a good breakdown of mono/di and all the terms!

FYI, it was during my NT scan which was at 13 weeks, I think, that they were able to fully determine chorionicity. The sacs have been clearly visible on each u/s so far; I go every three weeks for a quick check-in that is a low-res u/s to listen for heartbeats.
 

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