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Hi Jane. Congratulations to your sister

Deciding whether the twins are identical or not based on the placenta/sac can be dodgy. There will no doubt be a few members here who can tell you stories of this.

There are two layers of the placenta - the chorion and the amnion. The amnion is the inside layer (the sac), the chorion the outside (the placenta). Mono applied to these terms (monoamiotic, monchorionic) means that there is ONE, DI means two.

Non-identical (DZ) twins can either have totally separate placentas (apparently about 50%) or the placentas can fuse. This fused placenta (which is still Dichorionic - having two separate chorions or outer placentas) can look a lot like a monochorionic placenta (which happens with SOME identical twins). Only identicals can have a truly monochorionic placenta (but as I said this is hard to pick on ultrasound).

Identical twins are tricky. They can have any sort of placenta that they want, basically! It all depends on when they divide. Roughly ...

* division within 3 days of fertilisation - twins will be dichorionic (have separate placentas - but remember, these can still fuse and look like one!)
* between 3-9 days - monochorionic (one outer layer) but diamniotic (each twin has it own separate sac within that outer layer).
* 9-12 days - monochorionic, monoamniotic - sharing one sac.
* after 12 days (maybe 15 days) - conjoined twins.

That is all adapted from a book called Twins and Multiple Births, by Dr Carol Cooper.

Also have a look at
http://www.twins.org.au/infofortwins/factsandfigures.htm
http://www.amba.org.au/content/resources/zygosity.html

http://www.birth.com.au/class.asp?class=67&page=13

I hope I have not confused you more!

This message was edited by joanne2 on Saturday, 12 March 2005 @ 11:01 AM

Hi Jane,
I was told I was having identical twins based on ultrasound scans which showed one placenta and only one thin membrane separating the babies, instead of 2.
As it turned out, when they were born there were 2 placentas fused together and 2 separate sacs. So much for modern technology and ultrasounds every 2 weeks!!
I think the only way to tell with 100% certainty is once they are born - either by a check of the placenta or a DNA blood test. 25 weeks sems like a long wait, though!



Lisa
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.

I have identical twins with two sacs and two placentas!

I have identical twins with two sacs and two placentas!

Letitia 34
Dan 34
DD 6
DS 3
DS 1
DS 1
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Hi

When we had our first proper internal scan at almost 12 weeks to confirm how many were in there (initially they thought three but it was only two) one of the other things they looked at were my overaries and the sonographer said she was almost 100% sure we were having identical twins. A) because there apeared to be one placenta (but they are often wrong about that on ultrasounds). B) because there was one chrorion (outer membrane) and with a thin membrane seperating the two amnion (inner membranes) and C) from looking at my overaries there was only one empty follicle and I had only ovulated once.

So we had all the sure signs of MZ twins with embryos that split later rather than earlier. And as it turned out they were right. It was definitely one large placenta, we also had TTTS (a complication which only occurs in MZ twins who share a placenta) and have since had DNA testing as part of a research study.

But as the other wonderful twin Mums have already pointed out there is really no 100% sure way of knowing until after the babies are born.

I would hope her OB would be able to answer all her questions and will give her lots of information about twins, how they develop and possible complications. The web sites already mentioned are great and my favorite book is 'Twins - a practical and emotional guide to parenting twins' by Katrina Bowman and Louise Ryan. You can get it at most book shops and it covers everything from conception onwards and is really a great book.

Hope that helps

Kate

I would not rely on an OB to give up-to-date information about twins, as all of mine told me I was definitely having fraternal twins because of the sacs and placentas. Yet here I am with two identical little boys.

Unfortunately, a DNA test is the only 100% way to tell if they are identical or not (unless they are boy/girl twins obviously).

Good luck to your sister.

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This message was edited by Binna on Monday, 14 March 2005 @ 4:34 PM