Twin Terms Part III: Plumbing Arrangements

White line laying on top of baby's face is the amniotic membrane

We’re done with the simple nomenclature and off to the plumbing designations, which can be a little complicated, and has lots of lingo, which you probably won’t be familiar with the first time you hear it. Don’t worry though –you soon will be!
Dizygotic (fraternal) twins will always have all of their own structures within the womb, often referred to as ‘di-di’. Each has their own water filled play area called an amniotic sac. Each amniotic sac is contained within its own outer covering, or chorionic membrane. The chorion is synonymous with the placenta. So every fraternal twin will have its own placenta.
Monozygotic (identical) twins however, have several possible ‘womb-mate’ arrangements. Usually you will see this expressed or referred to with the lingo ‘mo-mo’ or ‘mo-di’. Those words can be translated so that the first ‘mo’ or ‘di’ refers to the twins’ chorionicity, and the second ‘mo’ or ‘di’ refers to the twin’s amniosity. (Note that is not animosity, which will certainly come into play around age 2 or so! LOL)
The chorion is a membrane that surrounds the amniotic sac (or sacs) and it is synonymous with the placenta. So if there is one chorion, there will be one placenta. If there are 2 chorions, each baby will have its own placenta. Monochorionic means that there is one chorionic membrane and that the babies will share a placenta. Sharing a placenta does generally put you at a higher risk for twin to twin transfusion syndrome where one twin gets more blood from the placenta than the other, causing variations in growth which pose a hazard to both twin’s health. But stress over it because there are many monochorionic multiples that do just fine! Your doctor will do lots of ultrasounds to monitor their growth and make sure they are growing at approximately the same rate.
The exception in this case is that there are cases where babies are dichorionic and have their own placentas, but as they grow, and as the placentas grow, they fuse together. Once the placentas fuse, it can be difficult to determine if your twins actually shared a placenta or each had their own which fused together. So, if you find out you are having twins at your 20 week ultrasound, and there only appears to be one placenta, you may not know the true chorionicity of your twins unless you have the placenta examined after their birth.
Amniosity refers to whether or not your babies have their own amniotic sacs. If your twins are monoamniotic, (‘mono’ meaning 1), they have only one amniotic sac and share a swimming pool together inside you. If they are di-amniotic, (‘di’ meaning 2) that means there are two sacs and each baby has it’s own pool of water which is separated from his siblings by a curtain-like membrane.
Unfortunately, sharing one amniotic sac does have its risks with the possibility of cord entanglement, which can. For this reason, it is normal for your physician to admit you to the hospital for constant monitoring of the babies at the point which they are viable (able to live outside the womb), which is around 20 weeks. The reasoning is that they want to monitor the babies for distress, so that at the first sign of trouble (likely resulting from cord entanglement) they could deliver the babies and save their lives. Note that this is not the same as bed rest that was indicated because of a risk for preterm labor. It is mostly for monitoring purposes. If your physician does NOT advise this, I would seek a second opinion.
The number of placentas and amniotic sacs can best be determined by ultrasound at around 12 weeks. At that time, the placentas and amniotic membrane are more highly visible than they are early in pregnancy when they are very small, or later in pregnancy when placentas can potentially grow together.
Now, if you are having triplets or more, you could have any combination of shared or individual placentas. So two of the babies could share a placenta and the third have its own, or all three share one placenta, or each of the three have their own placenta. Same thing with amniotic sacs – some babies could share a sac while others have their own.
Ok, if you fell asleep during that, you are probably not alone! So here is a brief review!
Fraternal Twins = Dizygotic twins (originating from 2 separate eggs and sperms) Fraternal twins will ALWAYS be ‘di-di’, meaning dichorionic and diamniotic having each their own placenta and amniotic sac.
Identical Twins = Monozygotic Twins (originating from the same egg & sperm which divided into 2!) Identical twins can be ‘mo-mo’, ‘mo-di’ or ‘di-di’. That means that even if your twins have their own placenta and their own amniotic sacs, they could still be identical and the only way for you to learn that they are NOT identical would be to rule out monozygosity with one of the following tests:
1.    Babies are different sexes, therefore not identical (duh!)
2.    Babies have different blood types, therefore not identical.
3.    Babies have a DNA test which compares their DNA and determines zygosity.
I hope you studied well..there may be a quiz later!

Twin Terms Part II: Identical & Fraternal

This is one of the most pondered concepts of twinning and it lends to the unique insight of the answer to the 'nurture vs. nature' question. This is why there are many studies done on identical twins - they provide a control group for genetic factors into disease and personality difference so that it is possible to have insight into how much of a disease is caused by genetics versus how much of it is attributable to an environmental element.

Ingrid & Helen (Identical Twins) age 2

Twins who came from the same fertilized egg are considered ‘monozygotic’, or ‘identical’ twins. They are begotten of 1 egg and 1 sperm, which after joining and growing for a time, divided in half, creating two blastocysts. There are a few exceptions to the above traditional origins of monozygotic and dizygotic twins, but they are mostly related to genetic mutations that are not common, and I won’t go into explaining them. If you want more info though, check here (see 'Half-Twins' Heading).

Helen & Ingrid age4

Generally, identical twins look very similar, though they may not look exactly identical.  My first twins were monozygotic, but I didn’t know it. They had slightly different shapes to their faces, but they looked more and more alike the older they got which led us to get them DNA tested. As they got older, other apparent differences were in their unique voices and facial expressions. My youngest, were also monozygotic, and they always looked alike it too, but they did have a slightly different shape in their ear which helped us tell them apart!

Dizygotic twins are no more genetically alike than any brother and sister. They originate from 2 separate eggs, which are fertilized by 2 separate sperm. That is why you should never ask a twin mom who has boy/girl twins if her twins are identical! (yes, some people do ask this!) some interesting exceptions to this include the possibility that fraternal twins could actually be the children of two different fathers! (Though I don't recommend this!)


One interesting note is that even though monozygotic twins are referred to as 'identical' twins, thier DNA is not completely identical. This is because there is a small amount of genetic material that is stored in the plasma of cells (not the nucleus where the most DNA is). So when the newly fertilized egg divides, one twin will end up with some mitochondrial DNA from the cell plasma and the other side will have the rest of it.

Ingrid & Helen age 5

A short delay...

Hi Guys,
Sorry for the lack of posts...soemthings come up this week and last, but I'll be back on track next week and try to make up for it!
Thanks for being patient!

Twins =Terms. Part I: Who's first?

As soon as you walk though the double doors of the twinning community (pun intended), you start hearing lots of befuddling words that describe the various possibilities of number, condition, and location of multiples in uetero. “Baby B, Baby 3,  dizygotic, monochorionic, transverse, Vertex, and even ‘mo/mo” what do they all mean? This is your all-expense paid trip to find out ‘what’ your babies are.



 Baby A, meet baby B, and perhaps Baby C and D too!
The first medical terminology you come across is likely to be during your first ultrasound. To differentiate your babies throughout pregnancy, they are labeled with the letters of the alphabet. Creative, eh? The baby who appears closest to your cervix is given the letter ‘A’ anticipating that he will be the first born. B is the second place winner and C the runner up.

Because the letters are used to distinguish one baby from the other, they will NOT rename them even if the babies (knowingly) switch places and one wrestles the other away from the cervix. However, if you have an early ultrasound, say 6 weeks and do not have another for a few months, it is very difficult to know that the baby that is A at the later ultrasound was actually the baby labeled A at your initial ultrasound. This is because the babies positions can change a lot and it is difficult to identify placenta location on early ultrasounds. Location of the placenta for each baby gives those ultrasound techs the best clue at who was A last time and who was B, because even though the babies can move, the placentas can not! Thank goodness that unless there is a medical concern, it is also irrelevant who was labeled who. (Except for us sentimental types who are wondering which book to scrapbook the ultrasound photo in!)

This can also make the end of pregnancy interesting as if they DO switch spots, your doctors and nurses start to get very confused at who is A and who is B – especially at the hospital. It also causes very interesting conversation with coordinating physicians who are appalled that you are going to attempt vaginal deliver when 'baby A' is not vertex.



 So this brings up an important point in medicine that applies to everyone who ever seeks medical care or treatment: Always know everything about your condition and pay attention to what is happening to you during your care. If you are supposed to have your left kidney removed, take a permanent marker and write on your back “Keep in” on the good side and “Take this one” on the other side. Folks, it happens too often. I am not joking about this – use the markers on your body. Ok, enough of a side rant.

After the babies are born, they receive new names. No, I’m not referring to the ones YOU will give them. Rather, they are warmly welcomed into the tradition of our modern society with the caring designation of (you guessed it!) a number. The first born is called ‘Baby  1’, the second, ‘Baby 2’ and you can guess from there. This is regardless of what their letter designations were. The baby who was born second will always be ‘Baby 2’ even if she was ‘baby A’ in utero. [Can’t help but bring images of Star Trek Voyageur to my mind ‘My designation is 7 of 9’. That’s what happens when you’re married to Mr. Geek.]