Originally Posted by labortrials
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.