Mono-Di & Di-Di

What does mono-di & di-di twins mean?

By. Shannon Klipstein 

Just a quick education for anyone interested. There are a lot of terms thrown around that can be confusing to understand so Im going to sum some of that up in this note.

First and foremost people who have had TTTS all have one thing in common. We all have identical twin or triplet pregnancies. Which means our pregnancies started with one single egg that split into two or three.

A Zygote= An egg and Mono, Di and Poly= the number of eggs

Monozygotic =Twins resulting from the splitting of one egg. Only 25% of Twins are Monozygotic
Dizygotic – multiple fetuses produced by two seperate eggs aka Fraternal twins. These can be hereditary and 75% of Twins are Dizygotic.
Polyzygotic – multiple fetuses produced by two or more eggs. This would be triplets, quads. etc that are all formed from their own sperate eggs.

Identical Triplets are also considered Monozygotic except they split into three and 1.58% of multiples will split into 4.
There is only one case where one egg split into 5 and there are no more than that recorded in history.

Identical Multiples are NOT the result of fertility drugs.
Identicals are always the same gender. Boy and Boy or Girl and Girl.

Sometimes with triplets or higher you can have one set of identicals plus a third baby from a seperate egg that is fraternal. In which case you could have a Boy/Boy or Girl/Girl and the fraternal triplet can be of a different gender.

So that take cares of the first part of “mono-di” twins phrase. So in my case my twins were “monozygotic” resulting from one egg and therefore identical girls.
Now to explain the “di” after mono.

This refers to the number of sacs in the uterus.
Diamniotic refers to the fact there are two seperate sacs the babies are in. This is the result when the egg splits between 4-8 days after conception.
If they are Mono-Mono they are both also in the same sac which is even more dangerous than Mono-Di Twins. This happens when the egg splits between 8-13 days after conception. But if they split between 13-15 days the result will be conjoined twins.
Fraternals would be considered Di-Di.. like Dizygotic and Diamniotic… they both have their own placenta and their own sac. With Mono-Di they share a placenta but both have their own sacs. 30% of Identical Twins are also Di-Di.  In extremely rare cases the two placentas can fuse together and cause TTTS.

In 20% of Mono-Di or Identical Twins their circulatory systems intermingle at random and create connections between the babies blood vessels. This can cause a disproportionate blood supply aka Twin to Twin Transfusion Syndrome.

If one twin dies in utero, blood accumulates in that twin’s body, causing exsanguination of the remaining twin.
(This is what happened to our twins) This means that the blood gets sifened/sucked out of one twin and rushes to the other twin causing the surviving twin to also pass away. Another complication that can happen, would be if one twin passes away and the other twin is still alive, the toxins from one can cause brain damage to the surviving twin. This is why fetoscopic surgery to laser the conjoining vessels is necessary.

Now about laser surgery…
(phew this is a lot of information to sum up to people in a breif sentence when asked “what happened to your twins”)

A fetoscopic surgery is performed (A scope is put into the womb and a tiny laser is put through the same hole) They guide the laser to which vessels they need to zap/disconnect/laser by looking at what the camera puts on the screen. If all goes well the twins will no longer be connected by blood vessels and have a greater chance of surviving. They will still have whatever % of placenta they originally shared but will no longer be exchanging blood. So one twin will still get more/less than the other. So even with a perfect surgery most TTTS babies are born premature and end up in the NICU with complications (that is a whole new can of worms when talking about the complications they can have in the NICU and their chance of survival after being premature… there is some great info on the March of Dimes website about the chances of survival at each gestational age and maybe I will make another note about that next time)
When you have a fetoscopic laser surgery you are told that there is a 90% chance one twin will survive surgery and there is only a 65-80% chance both will survive. This is where a lot of people end up with only one surviving twin. And in extremely rare cases (like with Anna and Claire) they will hit an artery during the surgery which causes excessive bleeding into the amniotic fluid in which case they won’t be able to see the placenta on the camera screen. They can slowly replace a little bit of amniotic fluid with saline but when there is too much they cannot get the fluid clear enough. (I had a 2pint blood transfusion to show how much blood was in there) This blood forms a clot which then keeps getting stuck at the end of the scope and they have no choice but to end the surgery. They cannot make the incision bigger on the uterus and they can’t get a blood clot of this size out through the tiny hole. So you have to wait til the blood clot is gone until you can retry surgery. So in our case (and this is going to sound super gross but it almost would have saved their lives) the blood clot that was floating in Anna’s sac was most likely swallowed by her. So now that the clot was gone we were able to have a second surgery. Thats when I became the first person at St. Joseph’s Hospital in Milwaukee to ever have the bleeding complication twice. So we were never able to seperate their connections. Claire passed away (because she was barely hanging on from kidney failure and other problems resulting from TTTS) and when she passed away, all the blood was exsanguinated from Anna and she died also.

This is super long… And I will continue to try and explain TTTS another day.
Note to self- Explain what a recipient and donor is and the health problems with each.
Explain the placenta share
hmmm….. to be continued….

This is the website I found when I was on bedrest that shows the survival rates for preemies and what complications they can have in the NICU:

http://www.marchofdimes.com/baby/premature_indepth.html

  1. Kelly
    October 14th, 2013 at 02:43 | #1

    Actually, I think you may have missed a term in there. Indeed, you are correct that identical twins are monozygotic (a single fertilized ovum that has divided) and fraternal twins are dizygotic (two ova fertilized by two different sperm).

    However the term mono/di does not refer to zygotes but is a reference in fact to monochrionic/diamniotic. The chorion being the outer membrane and placenta, the amnions being the sacs. The term di/di, again, does not refer to the zygotes but is an abbreviation of dichorionic/diamniotic.

    As you stated, identicals can be di/di (dichorionic/diamniotic) but will always be monozygotic -never dizygotic.

    I just wanted to be certain that the correct terminology is out there, as it could confuse some people otherwise.

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