Nov. 1, 2021

048 Types of Twin Pregnancies

048 Types of Twin Pregnancies

Different Types of Twins: Monozygotic & Dizygotic

There are two main types Types of Twins twins when it comes to pregnancy, dizygotic twins and monozygotic twins.  In this episode, we review these different types.

Types of Twins


There are two types of twins: Monozygotic and Dizygotic. Each has its own subcategories. (Di) means two; (Mono) means one; Zygote is a fertilized egg. Dizygotic means two fertilized eggs, and Monozygotic means one fertilized egg that split into two. Dizygotic is commonly known as fraternal twins, which means non-identical twins. With Monozygotic, it would be identical twins that split into two.


You can have twins one of two ways. With Dizygotic, you can have a woman ovulate two eggs at the same time, which is uncommon. That means each ovary ovulates an egg, and they both find their way into the uterus and get fertilized, so you have two pregnancies going at the same time. Basically, they’re siblings, but instead of carrying two years apart, your body decided that you’re just going to carry two at the same time, getting a two for one. The other type of twins, which is less common for every two or three cases, is Monozygotic. This is where one egg gets fertilized for whatever reason.


With Monozygotic, there’s one egg that splits itself and turns into “Monozygotic Twins” or identical twins. If that egg splits very early, then each piece of the egg develops a completely separate pregnancy with a completely separate placenta.

Early Conception of Twins


If the egg splits within the first three days, you have two separate placentas and two separate pregnancies. With twins, there are two separate layers called the Chorion and Amnion. These are the two layers of the placenta. The one that is further inside is the Amnion, and the one that is on the outside is the Chorion. With normal pregnancies, both act as a single layer. But with twins that are completely separate, they will have two of both layers.




If the egg separates after the first three days, but not after the first seven days, then each baby will develop its own Amnion even though they will share a placenta. That means each baby has a cord running to the same placenta on different sides of the amniotic separation, which is that wall between them. That wall is a membrane that prevents the twins from entangling their cords. If the baby separates after that seven-day zone, then they not only do share a Chorion, but also an Amnion. This means that both Chorion and Amnion are shared by the twins.

Monochorionic and Dyamniotic Twins


Monochorionic and Diamniotic twins have special considerations and special risks. If it was actually two eggs, and they both get fertilized, then it’s Dizygotic. But if it happens to be a Monozygotic twin that developed early, it will act like a Dichorionic. So how do you know what you are dealing with? You got to look early because early on, it’s very clear on ultrasound if you got two eggs in there that got fertilized or one egg that split in half.


Even if you have one egg and one sperm that got fertilized that split into two, if they split early enough, they are going to have their own two pregnancies. It’s going to be their separate pregnancies. If there had been two eggs that get fertilized, it would act the same way. They will be genetically identical.


7% of Monozygotic twins are Monochorionic or Diamniotic.  But the majority of twins are Dizygotic, which means they come from two separate eggs that are fertilized early in pregnancy. With ultrasound, it’s very easy to see early on the two separate sacks that are almost not touching your or very little touching. There’s a sack, and there’s a baby on this side. Those sacks are going to grow, and they’re going to merge. If one placenta isn’t attached to each other or they are within two separate locations, then you are dealing with Diamnitoic twins.

Managing and Identifying Twin Pregnancies


To figure out what type of twin pregnancy a woman has, their placenta is looked at using ultrasound. The points that are looked at are if the twins are sharing a placenta or if the placenta is in the same place. Currently, you cannot tell on ultrasound if it’s one placenta or two that are just against each other. Sometimes, placentas grow to each other, and there won’t be any separation between them. At this point, it’s very important to know if it is Dichorionic or Monochorionic because each is managed differently.


Another way to identify a twin pregnancy is to look at the twin membrane. You then magnify that, and then you measure it. If it’s above point two millimeters, then it’s a sign that it’s a Dichorionic. But if it’s less than .12, then it’s a sign that it is a Mono.


Once you have techniques, you don’t get rid of them. Even if a better technology comes along, you will learn to accept it. But sometimes you can’t see it that well. If the patient’s hard to scan or the memory is not in line well, the best way is to scan the membrane, and you zoom in on it. You count the layers, and if you see two layers, that means that the baby has one outer sac and two inter sac. But if the baby has two outer sacs and two inter sacs.

Twin Transfusion Syndrome


When you have a Monozygotic twin, there is a possibility that one twin can thrive more in the uterine environment than the other twin. With Monochorionic twins, there is a 15% chance of something called Twin Transfusion Syndrome. This is where one baby is getting too much blood back from the placenta, and the other baby is getting too little. The continuous pump where one baby continuously gets more and the other one gets less makes the other baby struggle. One baby cannot keep up with the volume and is overloaded, while the other one is struggling because it’s not getting enough blood.


If the twins are identified as Monozygotic and Monochorionic, then one of them is going to be given more blood. When blood goes to the placenta, it is shared equally between the twins. But if the arterial supply of the placenta is a bit different than the return, then both babies don’t get the same amount of blood. Instead of each baby having the same amount of return channels, both get different supplies of blood. One is automatically going to get less and the other one a little more. Then the sequence of Twin Transfusion begins. One baby’s getting overloaded, the other one’s not getting enough.


The earlier Twin Transfusion happens, the bigger the problem. If it happens late in the game, then you deliver the babies. But if it happens early in the game, you can’t deliver because they’re too premature, and they can’t survive. So if it happens early, you have to stop this vicious cycle from continuing. Most of the time, people will lose a baby because of the twin transfusion sequence if it’s left untreated because the fluid is so great. The excessive fluid will cause pre-term labor, and the patient will just lose the pregnancy because they go into labor early. Even if they carry long enough to get into viability, one baby will get heart failure because it can’t handle all the fluid.


**of note The correct term is reverse in this episode. The correct information is as follows: The fetus with more blood flow is called the recipient twin, and the fetus with less is called the donor twin



Velamentous Cord Insertion


Velamentous Cord Insertion is when the umbilical cord hits the placenta and branches into a spiderweb-like structure, and they drop into the placenta at different points. And that’s how all the blood is getting everywhere. But if that separation happens, then it can become a major problem. It’s a common complication in any pregnancy because you don’t want those vessels exposed, especially if they’re in front of the cervix. With twin pregnancies, Velamentous Cord Insertion is a more common phenomenon where you have that cord insertion. Often, it is tied to a twin transfusion sequence.

Treating Velamentous Cord Insertion


You can go in with a scope, and you can identify the vessels. It mainly involves a lot of ultrasound and targeting. Then you either ablate with a laser or radio waves. Oblate means you are sealing or coagulating them off. You are taking the vessels that run from A to B or B to A, and you are dividing the placenta, so it’s going to function like two separate placentas instead of one. If you overdo it, then you end up with the reverse where the other baby is getting too much. This is because you go in with instruments, and therefore there’s a risk of pre-term labor or amniotic fluid membrane rupture.

Monoamnionic Twins


Twin Transfusion doesn’t happen with Mono twins because there’s only one membrane on one side. The problem is that the cords are tangled, which happens early on. As soon as locomotion happens within 12 weeks, babies start swimming. This is the rarest and highest risk type of twin. What happens with those twins is you don’t do anything until 24 weeks because you can’t do anything. You just hope for the best. Once you get viable, you basically put them in a hospital and watch them, and you monitor them several times a day. Once you start seeing fetal heart rate decelerations, you get everything ready and get them delivered. If nothing bad happens, depending on how conservative you are in 34 weeks, just go in by C-Section and deliver them.

Conjoined Twins


The egg doesn’t split until day 11 or 12. If it doesn’t split within that period, you will get conjoined twins. That is a really big deal because now you have a team of surgeons figuring out if you could separate them and depending on where they’re separated. Sometimes they share vital organs, and you can’t separate them. But these conjoined twins happen very rarely.

Managing Risks for Twin Pregnancies


If you have monozygotic twins, but they’re Monochorionic and Diamniotic, then they have two separate rooms. In this case, you just have to monitor them every week.  You monitor them every other week because even the simplest, least complicated twins are high risk because there’s growth discordance where one baby grows better than the other. There is a risk of pre-term labor, which is much more common with twins. There’s also the risk of placenta abnormality, where the placenta is in the wrong place. There are all kinds of risks that go up, so you need to get ready for double risks.

Twins with Different Birthdays


Every once in a while, you might find twins that have different birthdays. These types of twin pregnancies are always C-Section. So whenever you encounter twins with different birthdays, it is assured that they were delivered through C-Section.

Least Popular Calendar days to deliver at Hackensack


The first date is September 11, the second is October 31, and the last one is a leap year, which is February 29.

Maternal Resources Social
|nstagram: @maternalresources

Subscribe to the podcast on Apple Podcasts, Spotify, Google Podcasts, & Stitcher and leave a review!