Hi Everyone!

I'll be the mod for this DB and I'm excited to see all your responses! 

Keep in mind that amnionicity and chorionocity are two separate things, and think about how they may relate to zygosity. I look forward to seeing all the great pictures and diagrams to help fully visualize what we would be seeing during OB scans.

-Karen

Hi everyone,

As we know by now, the chorion and amnion are two completely separate things. When it comes to twinning, some people may get the both confused when referring to them as di-di, mono-di, mono-mono, etc. 

When you have 2 fetuses within one amnion, they will share the amniotic sac, placenta, and will be identical. This happens when the zygote (mono in this case) splits after 8 days... if it doesn't split until after 13 days then it can cause conjoined twinning. You could also have diamniotic within one chorion (mono-di), where twins would have separate amniotic sac but share a placenta. 

When it comes to monozygosity, you could have a dichorionic pregnancy, in which case the amniotic sacs would be separated. This would happen if the zygote had split early, before the 8th day. On the other hand, dichorionic twins could be fraternal (dizygosity), when two ovums were fertilized and implanted separately. Dichorionics have separate chorion, amnion, and placenta. 

One of the most telling sonographic differences would be the Lambda or Twin Peak sign and the T sign. The Lambda sign would suggest a di-di pregnancy, whereas the T sign that is much thinner, because it contains only the amniotic membrane would suggest a mono-di. These are important because more complications arise from twinning, especially when they share the same little space and "utilities," as Dr Wilson said in today's lecture.

Sorry this image is so huge but it really illustrates everything well!

The differences between amnionicity and chorionicity begin at the cellular level. We know the chorion will develop from the trophoblast, also called the outer cell mass, and the amnion is formed from the embryoblast which is our inner cell mass! The chorion is the outermost layer and contains the chorion frondosum which will become the fetal side of the placenta, while the amnion lives within the confines of the chorion and is the cavity where our embryo will grow. 

To remember the difference between these two you must remember where they originate. Since the chorion is from the outer cell mass it is always the exterior portion, and the same is true of the amnion which comes from the inner cell mass and will be the most interior.

There are several different sonographic appearances we can rely on to help determine what type of twinning has occurred. While it might not always be possible to tell the zygosity we should be able to make the distinction between DC/DA, MC/DA, & MC/MA especially when scanning in the first trimester.

DC/DA twins should display the twin peaks or lambda sign in early pregnancy as a result of the 4 layers (2 chorion and 2 amnion) and the thick membrane between each embryo. MC/DA will have a much thinner membrane forming the t-sign from the single chorion and two amnion. MC/MA twins will not display a membrane between twin A & B. This is because they are enclosed in the same chorion and amnion which we should be able to see with sonography. 

Its important to know which type of twinning as each one presents with risks, some greater than others. If we see something that resembles a membrane in a pregnancy thought to be MC/MA we should worry for amniotic band syndrome, whereas if the pregnancy were MC/DA and we saw this it would be normal.

Amnionicity is the number of amniotic membranes that surrounds the fetuses in multiple gestations. Chorionicity is the same concept but with the chorion instead of amnion, so how many chorions are there surrounding the fetuses. I remember this because it has the different sacs right in the names! So it's really easy to remember which is which.

This can get confusing because we have Dizygotic where the fetuses are from 2 separate ova and Monozygotic where the fetuses are from one ova that split into 2. 

With dizygotic twins, they will always be dichorionic/diamniotic twins with their own amnion, chorion, and yolk sacs. They'll also have their own placentas, but they can sometimes be fused. 

Monozygotic twins are more tricky because theres 3 different outcomes. 

1. Dichorionic/diamniotic twins with 2 chorions and 2 amnions
   • This will have a thick intertwining membrane and a twin peak sign 
   • We can also use gender as a determining factor because a male and female have to be fraternal twins 
   • They live in totally separate houses- separate everything 
2. Monochorionic/diamniotic twins with 1 chorion and 2 amnions
   • This will have a thin intertwining membrane and a T sign 
   • They live in the same apartment building but in different apartments
   • Sharing a placenta but they have their own yolk sacs and amnions 
3. Monochorionic/monoamniotic with 1 chorion and 1 amnions
   • These fetuses live in the same apartment building and in the same apartment.
   • Shared placenta 

Mono/DI

Di/Di

Mono/Mono

Amnionicity refers to the number of amnions and chorionicity refers to the number of chorions. The amnion is the membrane that surrounds the amniotic cavity where the fetus lives. The chorion is the outer membrane surrounding the gestational sac which will help with developing the placenta. It helps me to remember that the chorion is the outer membrane because the chorionic frondosum and the decidua basalis are what connect and form the placenta.

The amnion appears as a thin membrane surrounding the fetus and amniotic cavity. The chorion is thicker and surrounds the chorionic cavity which contains the yolk sac. Sonographically we can see these different membranes and this helps with determining if the twins are monozygotic or dizygotic.

The lambda sign, or twin peak sign, is a sonographic finding that is seen with dichorionic diamniotic twin pregnancies. There is a thick dividing membrane between the gestational sacs made up of the two chorions and two amnions. A triangular projection of placental tissue between the sacs can also be seen. This can be seen with dizygotic and monozygotic pregnancies.

The T sign is a sonographic finding that is seen with monochorionic diamniotic twin pregnancies. The two amniotic membranes appear as a thin line that reaches up to the one placenta. This is seen with monozygotic pregnancies and raises the concern of TTTS because there is only one placenta. 

If a dividing amniotic membrane is not seen between the fetuses then they are likely monochorionic monoamniotic. This is also seen with monozygotic pregnancies and there is still the concern of TTTS with this type as there is only one placenta. These types of twins are also at risk of being conjoined because there is no dividing amnion.

I've attached an image that shows the amnion and chorion for a bit of a refresher.

With multiple gestations, we are concerned with the relationship between two sacs: the amnion and the chorion. The chorion is part of the gestational sac and is therefore related to the placenta. The amnion is a cavity within the chorion and is where the embryo will develop. The chorion is established prior to the amnion. 

Chorionicity is evaluated to determine if the twins share the same placenta (monochorionic) or have their own separate placentas (dichorionic). Dichorionic twinning is possible when there are two fertilized ova (dizygotic) or when one fertilized ovum (monozygotic) splits before day 4 post-fertilization. Dichorionic twinning can be identified by looking for the twin peak or lambda sign, ideally between weeks 10-14 of gestation. It looks like a thickened, triangular area of the placenta at the base between two chorions. The thickness of the separating membrane should be > 1.5 mm. 

Amionicity is evaluated to determine if the twins share an amnion (monoamniotic) or have their own amnion (diamniotic).  Diamniotic twinning occurs when there are two fertilized ova (dizygotic) or when one fertilized ovum (monozygotic) splits between days 4-8 of gestation. Diamniotic twinning can be identified by looking for the T-sign. This is a thin membrane that separates the embyros/fetuses. The thickness of the separating membrane should be < 1 mm. Monoamniotic twinning will lack the T-sign. 

Twin gestations are higher risk pregnancies than singletons. The risk increases as chronicity and amnionicity decreases, with monochorionic/monoamniotic being the most risky. 

The chorionicity and amnionicity combinations are as follows:

Dizygotic: dichorionic/diamniotic

Monozygotic: dichorionic/diamniotic, monochorionic/diamniotic, monochorionic/monoamniotic

Here is an illustration of the chorionic/amniotic combinations.

Below is a video that helped me in the few first weeks of this course. It talks about the gastrulation process, which involves the establishment of the chorion and the amnion. 

Although they may seem similar, the chorion and the amnion are distinct membranes, and they can be distinguished from one another sonographically. The amnion is the inner membrane that surrounds the embryo, and contains amniotic fluid within the amniotic cavity. The chorion is the outer layer that contains the amnion within it. 

Since we can have a DC/DA placentation with either a dizygotic twin or monozygotic twin, we cannot rely strictly on the number of chorions and amnions to determine the zygosity. We can look at the  genders to help us determine zygosity in this case, because monozygotic twins will always be the same gender, in contrast to dizygotic twins, which can be the opposite gender, or the same. So if we determine the twins are opposite genders, we know they are dizygotic (fraternal) twins. 

With that being said, although we can’t always determine the zygosity, we are able to distinguish between DC/DA, MC/DA,  and MC/MA twins. We can do this by identifying the number chorions, the number of amnions within the chorions, and the number of embryos within the amnions, along with some key sonographic signs. 

• DC/DA twins will have the “twin peak sign” aka “lambda sign”, which  will appear as a thick septating membrane, thickest at the placental surface, and tapers. It’s a result of the 2 chorions and 2 amnions. 

• MC/DA twins will display the T-sign, which will appear as a thin membrane, which is formed by the 2 amnions. 

• MC/MA twins will lack an obvious intertwin membrane since they share 1 chorion and 1 amnion. 

It’s important to identify which type of twinning we are dealing with, because it will predict the adverse outcomes in the perinatal period. We know monozygotic twins are at a higher risk for abnormalities and adverse outcomes. We also know MZ twins have different levels of perinatal mortality risk depending on their placentation. MZ DC/DA have a 10% risk and are associated with IUGR, MC/DA have a 25% risk and are associated with TTTS, and MC/MA have a 50% risk. The many complications associated with twins makes it crucial that we as sonographers know how to determine the type of twin, because it will greatly affect the type of care they will need.

I found this video lecture nice just to reinforce what we’ve already gone over and touch on some additional things like "twin mimics" to watch out for when scanning!

As we know the amnion is the innermost part of the placenta. It lines the amniotic cavity and holds the amniotic fluid and the developing embryo. The chorion, on the other hand, is the outer membrane that surrounds the amnion, the embryo, and other membranes and entities in the womb.

It helps me to remember that the amniotic fluid is surging the baby, therefore the amnion would be the innermost portion and the chorion is the outermost portion.

When differentiating what kind of twins I'm dealing with sonographically, I can look for the T sign or lambda sign which helps differentiate which I'm dealing with. T sign typically indicates monochorionic, diamniotic and the lambda sign typically indicates dichorionic, diamniotic.

hey all!

When I think of chorionicity I always think the outer layer of cells or the trophoblast cells that the blastocyst creates and the layer that fuses with the mothers endometrium (decidua basalis) to create the placenta. Then there is the amnion which is the inner embryonic membrane that encloses the fetus. To me the amnion is basically the second layer of protection that separates the fetus from its food supply. With all that said here is how I like to break it down when it comes to twins.

First I needed to understand the days after fertilization

2-3 days = Morula formation 

4-6 days = Blastocyst w/inner cell mass / Chorion  

7-8 days = Implantation 

8-9 days = Amniotic Cavity

2 different ovum + 2 different sperm = 2 different zygotes

2 different Morula 

2 different  blastocysts

2 chorions form therefore 2 different placentas

By default they have to have their own amnions 

Monozygotic/ di/di

1 ovum + 1 sperm = 1  zygote

Splits < 3 days = Separation in the morula formation or cleavage stage 

2 separate Morula 

2 separate blastocysts, 2 inner cell masses

2 chorions form therefore 2 different placentas

by default they have their own amnions

Monozygotic mono/di

1 ovum + 1 sperm = 1  zygote

1 morula 

1 blastocyst, 1 inner cell mass

1 chorion = 1 placenta

Splits day 4-8 (inner cell mass separates in blastocyst stage)

1 Chorion & 2 inner cell masses

1 Chorion & 2 amnion for each inner cell mass

Monozygotic mono/mono

1 ovum + 1 sperm = 1  zygote

1 morula 

1 blastocyst/1 chorion = 1 Placenta 

1 Chorion & 1 inner cell mass

1 Chorion & 1 mnion around the inner cell mass

Implantation 

Embryo splits days 8-13 (separation in double layer embryo)

Incomplete Separation

all of the above occurs however the by day 13 the embryo never fully split and are fused. 

Here are some good images of the sonographic T -sign vs. the Lambda sign

If the twins are dizygotic they should always have the lambda sign = dichorionic/ diamniotic

If the twins monozygotic and have the lambda sign = dichorionic/ diamniotic 

If the twins are monozygotic and have the T-sign = monochorionic/ diamniotic 

If the twins are monozygotic and have nothing separating them = monochorionic/ monoamniotic 

These are important to understand and know how to identify them because we need to know if they share a blood supply/ placenta and have vascular connections

I find metaphors helpful, and this article really helped me understand chorionicity and amnionicity:

https://jdc.jefferson.edu/cgi/viewcontent.cgi?article=1000&context=rsfp

It uses "The Apartment Analogy" similar to Dr. Wilson's example of houses on the same street. 

For me, it helps to break things down into the simplest terms.  The chorion, as we learned previously, is the outer membrane and is associated with placental formation. So chorionicity is going to determine the placental number, and therefore if the fetuses are sharing resources. If a gestation is dichorionic, it has to be diamniotic (otherwise you will have an empty gestational sac). So if you see the lambda sign, you know there are multiple membranes between the fetuses, illustrating both amniotic and chorionic layers.

If there is only one chorion, you can still have two amniotic sacs (monochorionic-diamniotic). These are still two separate living situations, but the wall between them is just amniotic membrane and therefore much thinner. This appears sonographically as the "T-sign".

The closest living situation monochorionic-monoamniotic, where they literally share a room.

Because twins have a higher risk of complications in monochorionic situations, it is important to accurately document amnionicity and chorionicity in multiple gestations.

Chorionicity and amnionicity are two very important concepts when it comes to assessing multiple fetal gestations. Knowledge of these concepts allows a Sonographer to understand the type of multiple gestations, and with this information treat each one accordingly. Chorionicity refers to the number of chorions, in other words the number of fetal connections to mom. Amnionicity refers to the number amniotic cavities present. Each amnion has amniotic fluid that is essential to the fetus’s growth and development. I remember chorionicity as the one on the outside and the amnion as the one on the inside. The sonographic appearance varies on the type of multiple gestations. In the case of dichorionic, we should see two chorions and two amnions (diamniotic), which can represent fraternal twins of the same or opposite sex or identical twins. For monochorionic we should see one chorion and one (monoamniotic) or two amnions (diamniotic), which represents identical twins of the same sex. Chorionicity and amnionicity can increase as the number of fetuses increase. This information is important because monochorionic twins have vascular anastomoses of some sort, which means something can occur that affects the amount of nutrients each fetus receives. Conjoined twins may also occur when the zygots spilts after 13 days. Conjoined twins are monochorionic monoamniotic, and share vascular connectivity.

  Amnioncitiy is the number of amnionic (inner) membranes that surround fetuses in a multiple pregnancy. Chorionicity is the number of chorionic (outer) membranes that surround the fetuses in a multiple pregnancy. chorionicity is best determined between 6-14 weeks gestation.

There are three types of twinning and these sonographic appearances help determine what twins you're dealing with:

Dichorionic-diamniotic- Twins who have their own chorions and amniotic sacs. They usually don't share a placenta and can be fraternal or identical depending on gender. With Dichorionic twins there should be a thick septum between the fetuses. 

 

Amnioncity is defined as the determination of the number of twins fetal amniotic membranes 

Chorionicity is defined as the determination of the number of chorionic membranes adjacent to the uterus 

via OB textbook by S. Stephenson and J. Dmitrieva

It's important to differentiate two and remember that as we are scanning a multiple gestational pregnancy we must investigate if each fetus has it's own chorion and amniotic sac! What helps me remember the difference is how Dr. Willson referred to the relationship as the fetuses are "sharing a home and in different rooms" = one chorion and two amnions. 

Understanding the differences between amnionicity and chorionicity, then becomes a fun matching game with the different kind of twins that you may encounter! Its also important to note that chorioncity can predict the fetal adverse outcomes, just as monochorionc/ monoamniotic pregnancies have vascular anastomoses. The different sonographic appearances that can help indicate what type of twins we are dealing with is noticing that there is a lambda sign for dizygotic twins (dichorionic/ diamniotic) and monozygotic twins (dichorionic/ diamniotic). Although some differences seen would be the appearance of the T-sign for monozygotic twins (monochorionic/ diamniotic), however if there is nothing  seen that is separating them it can be still be monozygotic twins (monochorionic/ monoamniotic). It is important to understand what kinds of twins that you are dealing with to keep a check up about how the fetus are developing or what to even expect for the fetal outcomes. As even monochorionic twins (image b) have about three times the mortality rate as dichorionic twins (image a).

Amnionicity is the number of amnions that are seen around the fetuses of a multiple pregnancy, and chorionicity is the number of chorions seen in a multiple pregnancy. I remember the difference by remembering that the amnion will be the one enveloping the amniotic fluid, not the chorion, so the amnion is the inner membrane and chorion is the outer.

We will see the twin peak, or lambda, sign when we have dichorionic/diamniotic twins, which shows the thick membrane between the two fetuses with 4 layers, each having their own chorions and own amnions. These can come from either monozygotic or dizygotic multiples. 

With monochorionic/diamniotic twins, we will see the "T" sign which is seen as a thin membrane of amnions meeting the chorion/placenta in a T shape (possibly perpendicular) instead of the triangle shape seen with di/di. 

The last amnionicity/chorionicity we may see is mono/mono, where there will be no membrane between the to fetuses seen since they share the same amnion as well as chorion. This and the "T" sign are both important to note since they both mean the pregnancy is monozygotic, which has a higher risk of fetal abnormality. Monochorionic twins will also all have a vascular anastomoses, so it is important to know if they are monochorionic so that their growth can be closely compared to see if one baby is receiving more blood/flow/nutrition than the other.

The last way I can think of to determine the kind of twins is if you see twins that are opposite sex. Fraternal twins will always be dizygotic and therefore dichorionic/diamniotic since they obviously were created from two separate ovum/sperm fertilizations.

The picture I'm copying is NOT from a medical source but I have a question about it... I found it when trying to find some images of di/di twin gender and its actually captioned as a gag gift twin picture. So my question is, if this photo is labeled correctly with twin A being a boy and twin B being a girl, shouldn't we see a amniotic/chorionic membrane between the two fetuses since opposite sex twins must be di/di? Or is the membrane not visualized in every plane, or are the genders maybe inaccurate? 

Hello cuties,

As we know, chorionicity has to do with the chorionic membrane, which eventually develops into the placenta, and amnionicity has to do with the amniotic membrane, which contains amniotic fluid. The chorionic membrane is the thick membrane surrounding the gestational sac and the amnion is a thin membrane. So let's start off in the beginning of it all: fertilization. A fertilized ovum is a zygote and the day when the zygote divides is what determines the amnionicity and chorionicity of a multiple gestation pregnancy. 

Fertilization starts 24 to 36 hours after ovulation and the sperm enters the ovum. The rapid division of cells, known as cleavage, occurs 24 to 30 hours after fertilization and continues to divide as the zygote travels through the fallopian tube. The cells keep on dividing and eventually become a ball of multiple tiny cells, called a morula and reach the uterus by day 4 where it starts becoming a blastocyst. The outer part of the blastocyst is called the trophoblast and will give rise to the placenta. The inner cell mass inside the blastocyst will give become the embryo. The blastocyst implants into the endometrium by day 7 and starts burying itself within the endometrium wall.

Now, if we are to follow this timeline, we can tell by when the zygote divides what type of pregnancy we will have:
Day 3= Dichorionic/Diamniotic

Days 4-7= Monochorionic/ Diamniotic

Days 8-13= Monochorionic/ Monoamniotic

Days 13+= Conjoined

When scanning multiple gestational pregnancies, we know to look out for the Twin Peak sign for a dichorionic pregnancy and the T sign for a diamniotic pregnancy. The twin peak sign is a triangular shape the chorions form when joining adjacent to each other and the T sign is the thin membranes of the amnions next to each other. The lack of thickness in the T sign confirms that the pregnancy is monochorionic.

Knowing what to look out for sonographically is important because there are different risks associated with different conditions. For example, monochorionic pregnancies have the risk of developing Twin to Twin Transfusions Syndrome (TTTs) and this can lead to cardiac failure and even death of the fetuses.

I like the discussion board this week because I feel like now that we are getting onto the third trimester I am forgetting the basis and what happens in the first trimester.  I have been sort of confused on when the amnionic cavity disappears and why we don’t see in the second and third trimester as often. The chorion is the outer membrane and the amnion is the inner membrane that the fetus lives in.  

Amnionicity is the thinner membrane you can have one or two amnions in twin pregnancies. One amnion then both fetuses would share one amniotic sac however if there was two amnions then each would have their own amniotic sac.

Chorionicity is the outer membrane and it's usually going to be thicker you can have one or two of these in twin pregnancies. They can have one chorion in which one or two amnions can live or there could be two chorion's with one amnion living in each. 

To best determine what kind of twins whether it is a fraternal or identical twin or whether it is a mono mono, Di mono, or di di twin pregnancy.  You might use gender to differentiate or you can use membranes.  There can be two kinds of membranes the lambda sign or the T sign and the lambda sign is going to be a thick barrier between both sacs demonstrating that there are diamniotic in dichorionic or there could be a T sign demonstrating that there is monochorionic diamniotic and usually this will be a thin line separating the two.  To be able to differentiate between what kind of twin pregnancy you're dealing with because of the pathology that might come with it.

Hi class,

I remember the baby is in the amniotic sac because baby floats in amniotic fluid. The chorion encapsulates the amnion. I remember this because it took me ages to understand embryology in the first few weeks, so I beat it into my head.

If I see a lambda sign I know I have 2 chorions and 2 amnions. If I see a T sign I know I have 1 chorion and 2 amnions. There can never be more chorions than amnions.

To tell if twins are mono/di or mono/mono look for the T sign. If that isn't seen try scanning through the amniotic cavity and look for a small echogenic line between the babies.

It's extremely important to know if the babies share a chorion or amnion for a ton of reasons. If they share a chorion there will always be some amount of anastomosis between the babies => this can cause one twin to be less developed than the other. If the twins share an amnion they run the risk of being conjoined.

Check out this article on conjoined twins. It's a bit long but just take a peek at the pictures if you can. https://www.sciencedirect.com/topics/medicine-and-dentistry/conjoined-twins

Sarah

Hi ladies!

Happy hump-day! We made it to mid-week. I don't know how but the weeks seem to drag yet fly by at the same time.

Gonna throw a little weird anomaly at ya... anyone heard of "fetus in fetu" before? What chorionicity/ amnionicity do you think these kinds of pregnancies would be for something like this to happen?

It's finally weekend, ladies!!

This discussion board topic was sweet and to the point. You all went above and beyond with posting.

We went over:

• Zygosity--
  • How many babies came from one ovum?
    • Monozygosity ALWAYS leads to same sex twins, IDENTICAL.
    • Multiple ovums fertilized will lead to FRATERNAL twins, may be same sex or xx and xy, having separate chorions
• Chorionicity--
  • Made of trophoblast cells
  • How many chorions in the pregnancy? 
  • Chorions house the amniotic sac(s)
  • Not possible to have more chorions than amniotic sacs
  • Lambda or Twin Peak sign
• Amnionicity--
  • Pouch of amniotic fluid that houses baby
  • Inner membrane
  • T sign

And as we know, timing has a lot to do with what type of twinning! For simplicity's sake we'll speak on 2 fetus twinning.

• If zygote splits < 3 days, it will be a dichorion, diamnion twinning.
• If zygote splits at ~4-8 days, it will be monochorion, diamnion twinning. These twins and any splitting after this point will have some sort of vascular placental anastamosis going on (TTTS, TRAP)
• If zygote splits at ~8-13 days, it will be monochorion, monoamnion twinning, really cramped quarters for babies.
• Any zygote that splits >13 days will result in monochorion, monoamnion CONJOINED twinning.

Suffice it to say that there will be a lot more important things to be looking out for when it comes to twins that share close spaces in mom! I'm glad we really hammered in some key points of what to look for between this and Leah's discussion board. I know just the amount of diagrams, explanations, and videos we've all pored over definitely made the information stick!

Thank you for a great discussion board. I hope more people will get to scan twins soon, including myself! Happy studying!

-Karen