Identical twins? - Mothering Forums

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#1 of 14 Old 02-06-2009, 02:32 AM - Thread Starter
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At my first ultrasound, they couldn't see two sacs. They saw two babies, but both in one sac. By my last ultrasound (10 weeks), they could see two sacs, but there was a very fine membrane between them. They both have their own placenta.

The doctor told me that technically, identical twins each have their own sac, but the membrane is much thinner, but he didn't mention if mine might be identical or not. Is there a possibility?
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#2 of 14 Old 02-06-2009, 12:23 PM
 
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It is possible, and it's not the thickness of the membrane that makes a difference.

There are four types of identical (monozygotic - or coming from one zygote) twins:

1) Di-chorionic, di-amniotic twin: twins that each have their own placenta and own amniotic sac. These twins result from the zygote splitting early.
2) Mono-chorionic, di-amniotic twins: twins that share a placenta, but each have their own amniotic sac. These twins split a bit later.
3) Mono-chorionic, mono-amniotic twins: twins that share both their placenta and their amniotic sac. These twins split very late - at least a week after conception.
4) Conjoined (or "Siamese") twins. Very, very late split.

Because it's possible for identical twins to each have their own placenta, it isn't always possible to know if your twins are identical before they are born. If they're sharing a placenta (and anything else!) you know that they're identical. If they're different sexes (one boy, one girl) you know they're fraternal. If they each have their own placenta and are the same sex, you can't know if they're identical until they are born. (And then you need to do a blood test and/or a DNA test to know for sure - identical twins don't always look alike at birth.)

I hope that helps!

(It amazes me how many doctors we hear about here who don't know this, or don't explain it well!)
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#3 of 14 Old 02-06-2009, 12:29 PM
 
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this should be a sticky !!!

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#4 of 14 Old 02-06-2009, 12:37 PM
 
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If they each have their own placentas, then is it still a "seperating membrane" that's seperating them? Can you even call it a seperating membrane - - seems to me it's just regular 'ole membrane coming off each placenta. I can't imagine how the thickness of a membrane coming off two seperate placentas would indicate whether twins are monozygotic.

OTOH, my identical twins had a very thin seperating membrane. There's still no way of knowing until they're born though. No one ever suggested that mine would be identical or fraternal while they were in utero (even though *I* knew they were identical
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#5 of 14 Old 02-06-2009, 01:57 PM
 
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Originally Posted by wendyjoe View Post
If they each have their own placentas, then is it still a "seperating membrane" that's seperating them? Can you even call it a seperating membrane - - seems to me it's just regular 'ole membrane coming off each placenta. I can't imagine how the thickness of a membrane coming off two seperate placentas would indicate whether twins are monozygotic.
The "membrane" separating the twins is either the walls of the amniotic sacs (for mono-di twins) or the walls of the amniotic sacs AND the walls of the chorions (for di-di twins). So, mono-di twins will have a "thinner" membrane - 'cause it's just the sacs and not the sacs and chorion.

So, it doesn't indicate identical twins, though it might indicate type.

I also understand that the shape of the separating membrane where it hits the placenta (a "T" shape) can indicate mono-di twins (as opposed to di-di twins). But I might have that backwards.

btw, seeing a membrane is a really, really, really good thing.

No membrane = nothing separating the babies = cord entanglement = over 50% mortality without intervention. And oh my head, the intervention. (So grateful it exists, so don't want to do it again.)

Also, you want to know if your babies are mono-di (sharing a placenta) because then you have to watch for TTS.

If they're di-di, you're golden. (Well, you know, except for that whole twin thing . . . )
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#6 of 14 Old 02-07-2009, 11:28 PM
 
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Sounds MZ to me. In early U/S some of the MZ babies look like they are in the same sac, most actually. Most MZ twins are monochoriatic, so they share the choriatic sac and a rare few share the amnion too. The membrane you saw was probably the amniotic sac membrane (very good to see!). If the Dr said anything about them sharing a placenta then chances are probable that you are having MZ twins. Do you have a pic we could scrutenize for you .

For example this is what MZ might look like at about 7 1/2 weeks.
http://i2.photobucket.com/albums/y9/...ins/Twins2.jpg

Lucia , Poly )O( Lactation Counseling mama lady.gifvbac.gifto 5 yo Goobersuperhero.gif and 3 1/2 yo MZ twins twins.gif Peanut and Sweetpea and 1yo Pumpkinbabyf.gif mmm placenta.gif
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#7 of 14 Old 02-08-2009, 01:41 AM
 
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You already got a good explanation so I won't go into that. My mz twins both had their own placenta and sacs and I was told numerous times they were dz. In that situation (same as yours) there is just no way to know for sure until they are born and then you can have them tested. Mine really looked pretty different at birth and I was very surprised they were mz.
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#8 of 14 Old 02-08-2009, 04:23 PM
 
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Originally Posted by shukr View Post
this should be a sticky !!!
YES! I think we seriously need a zygosity/chorionicity sticky STAT!

Betsy, mama to beautiful, strong MZ twins Lillian and Kate, born 11 weeks early on January 10, 2006.
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#9 of 14 Old 02-09-2009, 01:46 PM
 
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Originally Posted by Haaayley View Post
At my first ultrasound, they couldn't see two sacs. They saw two babies, but both in one sac. By my last ultrasound (10 weeks), they could see two sacs, but there was a very fine membrane between them. They both have their own placenta.

The doctor told me that technically, identical twins each have their own sac, but the membrane is much thinner, but he didn't mention if mine might be identical or not. Is there a possibility?

The people who've posted have given the goods on di-di and mono-di twins.

I'm just a little perplexed by the original post, though.

Two sacs but a fine membrane between them? That typically is the language used when the sacs (amniotic sacs) are inside one single chorionic sac. A single pregnancy would be one chorionic sac with an amnion inside it. It's several layers of membranes, kind of an outer & inner layer, but the amniotic membrane is the one that encloses the baby in the fluid.

When twins share a placenta (and therefore both are inside one single chorionic membrane), you typically will find them "in their own sacs" within that outer bag. That's when you have the "very thin membrane" between them because all that separates them is two layers of amnion....one layer for one twin, and one layer of the other twin's bag. This is the dividing membrane that sometimes isn't visible on ultrasound (if it's not a higher resolution machine.)

If they weren't in one chorionic sac, then the "membrane" separating them would be thicker because it would consist of both amniotic membrane and chorionic membrane.

I've never heard of any situation in which MZ/identical twins with TWO placentas (and therefore their own chorionic sacs) were separated by a "very thin" membrane. Their dividing membranes would be exactly the same as those of DZ/fraternal twins.

Unless your doc is talking only about the amniotic sacs, and the twins do NOT share a placenta. Then the very thin membrane would indicate something, because it would be only an amniotic membrane between them.

But it sounds like your doctor is trying to make a distinction between the thickness of the membrane separating DZ di-di twins and MZ twins who happen to be di-di. That makes no sense to me.
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#10 of 14 Old 02-11-2009, 12:43 AM
 
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Originally Posted by AmyC View Post
If they weren't in one chorionic sac, then the "membrane" separating them would be thicker because it would consist of both amniotic membrane and chorionic membrane.

But it sounds like your doctor is trying to make a distinction between the thickness of the membrane separating DZ di-di twins and MZ twins who happen to be di-di. That makes no sense to me.
Thank you. That's exactly what I was trying to say. Why even speak about "seperating membrane" when the twins don't share a placenta. Makes no sense.
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#11 of 14 Old 02-11-2009, 01:34 AM
 
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Originally Posted by vanauken View Post
(It amazes me how many doctors we hear about here who don't know this, or don't explain it well!)
They probably think it's too complicated for us mommies.

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Originally Posted by shukr View Post
this should be a sticky !!!
Totally!

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Originally Posted by wendyjoe View Post
No one ever suggested that mine would be identical or fraternal while they were in utero (even though *I* knew they were identical
I *know* mine are fraternal. And the peri, RE, and OB all assume that they are fraternal (or at least di-di MZs).

Quote:
Originally Posted by fyrebloom View Post
For example this is what MZ might look like at about 7 1/2 weeks.
http://i2.photobucket.com/albums/y9/...ins/Twins2.jpg
Oh wow, very different from what mine looked like around that point:
http://lh3.ggpht.com/_peG8VRxDZtM/SX...M0/s400/7w.jpg

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Originally Posted by AmyC View Post
But it sounds like your doctor is trying to make a distinction between the thickness of the membrane separating DZ di-di twins and MZ twins who happen to be di-di. That makes no sense to me.
I'm probably confused about what you wrote because this whole DZ/MZ thing is so strange and new to me... but this is sort of how it's been explained to me as well. Because my twins are separated by a thick membrane, the docs say it's most likely that they are fraternal twins. I don't think they're saying that other configurations AREN'T possibilities, just less likely if the separation between the twins is substantial?

So, all I know right now . . . is that they're in their own sacs and that they have their own placentas. And we might have spotted boy parts on Twin B today too. I really hope one of them is a boy though we'll be happy with just 2 healthy whatever kids!

Kimberly, mom & wife - blogging.jpg about pregnancy and birth
DD 2004; 3 angel1.gif babies 2007-08; rainbow1284.gif twin DDs 2009; DD 7/12/11 hospital uhoh3.gif VBAC bouncy.gifafter 2 cesareans!

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#12 of 14 Old 02-13-2009, 01:49 AM
 
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Originally Posted by labortrials View Post

I'm probably confused about what you wrote because this whole DZ/MZ thing is so strange and new to me... but this is sort of how it's been explained to me as well. Because my twins are separated by a thick membrane, the docs say it's most likely that they are fraternal twins. I don't think they're saying that other configurations AREN'T possibilities, just less likely if the separation between the twins is substantial?

So, all I know right now . . . is that they're in their own sacs and that they have their own placentas. And we might have spotted boy parts on Twin B today too. I really hope one of them is a boy though we'll be happy with just 2 healthy whatever kids!
But if they KNOW that there are two placentas, then the membrane between the babies is going to be "thick" because where they touch, there is baby A's chorion & amnion, and baby B's chorion & amnion. That's as many layers as there can be. And the same number of layers will be between di-di twins whether they are MZ or DZ.

The only time the thickness of the membrane (or rather, the thin-ness ) is "telling" is when there seems to be a single placenta. After a certain point in the gestation (I think after 13 weeks), it is no longer possible to confirm zygosity on ultrasound by counting layers of membranes. A "single" placenta may be fused, or it may be single. This is when the thickness of the dividing membrane can help to educate the guess somewhat.

If it is "very thin" and if the connection to the placenta is "very clean" and seems to form a sharp T, rather than kind of curved/layered edges, then chances are very good that it is a single placenta, or a mono/di pair. This is especially likely if the membrane was hard to see (if the possibility of mo/mo twins was suspected initially, and the mom was sent on to a perinatologist for a higher resolution ultrasound & consultation.)

At 14 weeks, I saw a perinatologist and he went over all those points (about the thin membrane between the twins, about the very clean T that seemed to be just a single layer amnion for each babe meeting, rather than extra membranes from chorionic material also being in that mix, about the apparent single placenta that didn't have any obvious characteristics of juncture or growing together.) He then commented that if we saw a girl and a boy on the ultrasound at 18 or 20 weeks, we'd know that we had been wrong about the educated guess.

He also mentioned that just a few weeks back, they'd have been able to literally count the layers of membrane and make a more certain conclusion.

But....if the twins are KNOWN to be di-di because there clearly are two individual placentas, it really doesn't make sense to talk about thinner membranes between identical di-di twins.

It's when trying to differentiate between a true single placenta and an apparent single placenta (but actually fused pair) that the "very thin membrane" actually becomes meaningful.
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#13 of 14 Old 02-13-2009, 01:50 AM
 
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Now, maybe they are assuming two placentas BECAUSE of the thicker dividing membrane, but they actually are fused so there's some question?
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#14 of 14 Old 02-13-2009, 02:40 PM
 
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But if they KNOW that there are two placentas, then the membrane between the babies is going to be "thick" because where they touch, there is baby A's chorion & amnion, and baby B's chorion & amnion. That's as many layers as there can be. And the same number of layers will be between di-di twins whether they are MZ or DZ.
OMG, Amy I'm so confused now! Aaaaaaaaaa! Or perhaps you're responding to the OP. I guess I could post a picture from the u/s for you folks to dissect. I find this so curious and interesting, but I am so spatially challenged, that talking about all of these layers and such is just confusing the bajeezus out of me. (And I've already read the naturallyparentingtwins info a gajillion times.)

Does anyone have pictures or illustrations that can help me understand this better. I'm obviously back to reading at the toddler level.

Kimberly, mom & wife - blogging.jpg about pregnancy and birth
DD 2004; 3 angel1.gif babies 2007-08; rainbow1284.gif twin DDs 2009; DD 7/12/11 hospital uhoh3.gif VBAC bouncy.gifafter 2 cesareans!

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