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Mono/Di Twins: Definitions, Risks, and More

Twins! Chances are you know at least a pair or two, but have you ever really considered just how they came to be?

After all, when you learned about the birds and the bees, it resulted in one baby. How is it possible for there to be two? (Maybe there’s something to that stork theory after all!)

If you’ve done any research on twins (or are now pregnant with twins of your own), you may have found that there are more terms than you expected: “identical” and “fraternal,” yes, but also “mo/mo,” “di/di,” or even “mono/di” twins.

You likely wondered: What do these mean, and how do these twins differ from other twins? Don’t worry — if you keep reading below, we’ll help it all make sense.

What are mono/di twins?

Short answer: monochorionic/diamniotic twins are identical twins who share the same chorion but have separate amnions while in the womb.

If you read that and said “Huh?” you’re not alone. The types of twinning are more complex than they first seem.

Keep reading for a deeper explanation, where we’ll define all those words.

Identical or fraternal

You may have heard twins referred to as identical or fraternal in the past.

Identical twins are those that share the exact same genes. Fraternal twins, on the other hand, share only half of their chromosomes, just like other siblings born separately.

This means that identical twins will always look the same, but fraternal twins can have different:

  • genders
  • heights
  • hair colors
  • eye colors

These twin types share different amounts of chromosomes because they’re formed in different ways.

Identical twins come from the same egg and sperm. The egg and sperm join together as in any conception, but the egg splits in two shortly after fertilization. Because they come from one egg, they’re sometimes referred to as monozygotic twins.

On the other hand, fraternal twins are sometimes called dizygotic twins — meaning, they form from two separate eggs. Fraternal twins are formed when multiple eggs are released around the same time, and each egg is fertilized by a different sperm.

Understanding chorions and amnions

While there are two large categories of twins (monozygotic and dizygotic), there are three potential types of twins based on what they share in the womb as they develop.

It’s possible for twins to share the same chorionic and amniotic sacs or have their own.

The chorion is the outermost membrane that connects the amnion, amniotic sac, and fetus to the placenta. Whether twins share this is important: One chorion equals one placenta but two chorions mean two separate placentas.

The more that twins share in utero, the higher the level of risk during the pregnancy.

Approximately of identical twin pregnancies are monochorionic (one chorion). The other of identical twins are dichorionic (two chorions), as are all fraternal twins.

The amnion is the innermost membrane that holds the fetus and amniotic fluid. Because the amnion is inside the chorion, it’s possible for the babies to share an amnion or have their own amnion, even if there’s only one chorion.

Still with us? The breakdown of the three potential types of twins based on various combinations of chorions and amnions is:

  • Mo/mo (short for monochorionic monoamniotic pregnancy).Mo/mo twins are monozygotic twins who share both the chorionic and amniotic sacs. In other words, there’s one placenta and one amniotic sac for both babies.
  • Mo/di (short for monochorionic diamniotic pregnancy). These monozygotic twins share a chorionic sac but have different amniotic sacs. To think of it another way, the big difference in this type of twin pregnancy is that there’s only one placenta. Each baby still gets their own amniotic sac.
  • Di/di (short for dichorionic diamniotic pregnancy).Di/di twins can be monozygotic or dizygotic. They each have their own chorionic and amniotic sacs. Although it’s a lot more cramped inside the womb, these twins are essentially developing just like they would if they were each being born as an only child.

How can you tell if you have a mono/di pregnancy?

Mono/di twins occur in about of spontaneous twins, and a mono/di pregnancy will be diagnosed by an ultrasound.

Ultrasounds are most accurate at determining chorionicity in the first trimester. So between 11 and 13 weeks, your doctor will be looking to see one placenta supplying blood to two fetuses.

At this point, your doctor will begin to look for whether there are two amniotic sacs. They’ll start to determine whether the amniotic fluid seems to be evenly dispersed between the two babies.

Once twins have been confirmed, your doctor may refer you to a high-risk specialist for the remainder of your pregnancy.

Because of some of the extra risks involved in a mono/di pregnancy, you can expect that you’ll likely need a fetal echocardiology in addition to routine ultrasound assessments.

The vast majority of mono/di twins are delivered without serious complications; however, even if no complications are identified before birth, mono/di twins tend to be born prematurely.

There’s also an increased likelihood of C-section birth or induced labor with mono/di twins in order to avoid risks.

What are the risks of mono/di twins? 

Twin pregnancies are considered higher risk because two babies are sharing a close space and the mother is carrying double the usual number of babies. That said, most mo/di pregnancies are uncomplicated.

Any twin pregnancies include an increased risk of:

  • Placenta previa. The extra baby weight in the placenta may cause it to hang lower or cover the cervix, known as placenta previa.
  • Placental abruption.Placental abruption happens because a larger and heavier placenta may be more likely to pull away from the uterine wall.
  • Placenta accreta.Placenta accreta occurs when the placenta embeds itself too deeply in the uterine wall.
  • Prematurity. Because of the space restrictions for twins as well as the higher risk of other complications, twins are more likely to be born premature, or before 37 weeks’ gestation. Your doctor may suggest an early induction or C-section.
  • Low birth weight. Since they’re more likely to be born early and have limited space to develop in the womb, twins are more likely to be born with low birth weight, less than 5 pounds 8 ounces.
  • Anemia. Twin pregnancies are a greater risk factor for iron and folate deficiency, which can lead to anemia.
  • Gestational diabetes.Gestational diabetes is when a person develops high blood sugar levels while pregnant.
  • Gestational hypertension.Gestational hypertension is when a person develops high blood pressure while pregnant. This may lead to preeclampsia if not treated.
  • Postpartum hemorrhage. Because of the large twin placenta or placentas, the uterus may have had to stretch more than normal during pregnancy. This leads to an increased risk of hemorrhage.

Mono/di twin pregnancies carry more risk than a di/di twin pregnancy because of the shared placenta.

In addition to the risks that all twin pregnancies face, mono/di pregnancies are at risk for complications related to a shared blood supply inside the placenta.

Circulatory imbalances can result in twin-twin transfusion syndrome (TTTS).

This occurs in approximately 10 to 15 percent of monochorionic/diamniotic pregnancies, when blood is not evenly shared between the twins. TTTS can lead to serious illness or death for one or both of the twins.

If signs of TTTS appear during ultrasounds, laser therapy may be used to coagulate the shared vessels on the surface of the placenta so that the blood is no longer shared. There’s also a heightened risk of premature delivery.

Another potential circulatory problem monochorionic twins can face is twin reversed arterial perfusion (TRAP) sequence. While this only occurs in approximately 1 percent of monochorionic pregnancies and 1 in 35,000 pregnancies overall, according to Children’s Wisconsin, it is serious.

In TRAP pregnancies, one twin develops normally while the other develops without a working heart and many other body structures necessary to function.

The two twins are joined by a large blood vessel, but the twin without a functioning heart receives all their blood from the healthy (“pump”) twin. This causes the healthy twin’s heart to work much harder than it normally would and can cause them to experience heart failure or death.

Twins sharing the placenta also risk unequal placental sharing (UPS). If during ultrasound assessments it appears that one twin is severely growth-restricted, your doctor will consider how far into the pregnancy you are and the risk of early delivery or other treatments.


If genetics isn’t your thing, this might be an overwhelming amount of information to take in — and that’s totally OK.

Unless you’re pregnant with twins, it’ll probably never matter whether someone else is a mono/di twin or a mono/mono twin. (Though now that you know more about it, you might be interested to find out!)

Remember, a variety of factors besides genes will determine the person someone becomes outside the womb. The experiences that shape people can be just as interesting and important to learn about as which parts of the womb they shared!


Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins

The purpose of this study was to evaluate the relationship between the number of yolk sacs and amnionicity in monochorionic twin pregnancies scanned early in the first trimester. We retrospectively reviewed images of all monochorionic twins scanned between 6 and 9.5 weeks' gestation and with pathologic or sonographic confirmation of chorionicity-amnionicity. Each film was reviewed for the number of yolk sacs present, as well as for the gestational age at which the amniotic membrane was first visualized. Twenty monochorionic-diamniotic pregnancies and two monochorionic-monoamniotic pregnancies met the criteria for inclusion in the study. In diamniotic pregnancies scanned at less than 8 weeks' gestation, only the yolk sacs were identified; none of the dividing amniotic membranes were detected. Two yolk sacs were identified in all but one case. In this case, although one yolk sac was seen at 6 weeks, follow-up scanning at 8 weeks revealed two yolk sacs. In each of the monochorionic-monoamniotic twin pregnancies, one yolk sac was seen at 9 weeks and a single amnion encircled both embryos. We conclude that the sonographic identification of two yolk scas in monochorionic twins enables us to make the diagnosis of diamniotic twins early in the first trimester, before the amniotic membrane can be imaged. The presence of one yolk sac should prompt a follow-up ultrasonogram to assign amnionicity definitively.

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A monochorionic diamniotic (MCDA) twin pregnancy is a subtype of monozygotic twin pregnancy. These fetuses share a single chorionic sac but have two amniotic sacs and two yolk sacs. 

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It accounts for the vast majority (70-75%) of monozygotic twin pregnancies although only ~30% of all twin pregnancies. The estimated incidence is at ~1:400 pregnancies 11.

An MCDA pregnancy results from a separation of a single zygote at ~4-8 days (blastocyst) following formation. These fetuses share a single chorionic sac but two yolk sacs and two amniotic sacs. By this time a trophoblast has already formed yielding a single placenta.

The layperson's term is that the twins are "identical" - in reality, they are phenotypically similar, and of course of the same gender. 

First trimester
  • shows a twin pregnancy with a single gestational sac, and almost always two separate yolk sacs 9,10 (differentiating from an MCMA pregnancy)
  • at 14-18 weeks, often a single placenta is seen: differentiating from a DCDA pregnancy
  • a thin inter-twin membrane may be seen
    • due to amnions abutting the placenta
    • present: differentiating from an MCMA pregnancy
    • but appears very thin without intervening chorion (often taken as <2 mm): differentiating from a DCDA pregnancy (although this assessment becomes increasingly difficult with the progression of pregnancy)
    • T-sign of the intertwin membrane
Second and third trimesters

Findings noted on a second-trimester scan include:

  • the number of placental masses, thickness of the membrane, and the presence/absence of the twin-peak sign are still viable options for determining chorionicity
  • fetal sex
    • almost always the same sex
    • in rare circumstances, postzygotic non-disjunction can occur

Negative findings:

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Treatment and prognosis


Potential complications that can occur with this type of pregnancy include:

Quiz questions



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  • 2. Wladimiroff JW, Eik-Nes S. Ultrasound in obstetrics and gynaecology. Elsevier Science Health Science div. (2009) ISBN:0444518290. Read it at Google Books - Find it at Amazon
  • 3. Nair M, Kumar G. Uncomplicated monochorionic diamniotic twin pregnancy. J Obstet Gynaecol. 2009;29 (2): 90-3. doi:10.1080/01443610802646793 - Pubmed citation
  • 4. Lewi L, Gucciardo L, Van mieghem T et-al. Monochorionic diamniotic twin pregnancies: natural history and risk stratification. Fetal. Diagn. Ther. 2010;27 (3): 121-33. doi:10.1159/000313300 - Pubmed citation
  • 5. Lewi L, Van schoubroeck D, Gratacós E et-al. Monochorionic diamniotic twins: complications and management options. Curr. Opin. Obstet. Gynecol. 2003;15 (2): 177-94. doi:10.1097/01.gco.0000063539.93768.06 - Pubmed citation
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  • 8. Merz E, Bahlmann F. Ultrasound in obstetrics and gynecology. Thieme Medical Publishers. (2005) ISBN:1588901475. Read it at Google Books - Find it at Amazon
  • 9. Bromley B, Benacerraf B. Using the number of yolk sacs to determine amnionicity in early first trimester monochorionic twins. J Ultrasound Med. 1995;14 (6): 415-9. J Ultrasound Med (abstract) - Pubmed citation
  • 10. Murakoshi T, Ishii K, Matsushita M et-al. Monochorionic monoamniotic twin pregnancies with two yolk sacs may not be a rare finding: a report of two cases. Ultrasound Obstet Gynecol. 2010;36 (3): 384-6. doi:10.1002/uog.7710 - Pubmed citation
  • 11. Barigye O, Pasquini L, Galea P et-al. High risk of unexpected late fetal death in monochorionic twins despite intensive ultrasound surveillance: a cohort study. PLoS Med. 2005;2 (6): e172. doi:10.1371/journal.pmed.0020172 - Free text at pubmed - Pubmed citation

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Twin-to-Twin Transfusion Syndrome and Fetoscopic Laser Surgery

Monochorionic, Diamniotic Twins

What are monochorionic, diamniotic twins?

Monochorionic, diamniotic (MCDA) twins are the product of a single fertilized ovum (egg), resulting in genetically identical offspring. MCDA twins share a single placenta (blood supply) but have separate amniotic sacs. The occurrence of MCDA twins occurs at a rate of three to four in 1,000 live births.

How is this condition managed during pregnancy?

MCDA twin pregnancies are more frequently monitored throughout pregnancy in order to maximize the chances for a good outcome. In addition to complications such as preterm labor and preterm delivery that all twins face, MCDA twins are at risk for complications specific to a shared blood supply. Circulatory imbalance across blood vessels linking twins places them at risk for twin-twin transfusion syndrome (TTTS). Uneven division of the placenta may result in unequal placental sharing (UPS). Given an association between MCDA twins and physical malformations, MCDA twins undergo fetal echocardiography in addition to usual ultrasound-based assessments.

How is this condition managed after delivery?

Though MCDA twins are at a higher risk for serious complications when compared to singleton pregnancies or dichorionic (fraternal) twin pregnancies, the majority are delivered without any significant complications. MCDA twins are usually born prematurely, even if no complications are identified before birth.


Twins ultrasound modi

Twin Chorionicity Explained

The terminology associated with a twin pregnancy can be confusing and overwhelming. Finding out that you’re having twins is just the beginning of an exciting experience. Along with adjusting to the idea of having two babies at once, there is a lot to learn. Here's a guide to understanding these terms, and what they mean for the developing fetuses.

During a twin pregnancy, you may have heard the terms “Di/Di” or “Mo/Di” used to describe twins. These terms are used to classify and explain twin anatomy in the womb. Although it may sound like a lot of medical gobbledy-gook, they're important concepts for parents because some twins are at risk for complications.

Twins in the Womb

Let’s start by understanding the environment. In the womb, a fetus grows within its mother’s uterus. The placenta attaches to the wall of the uterus and serves the dual purpose of removing waste and providing nutrients to the fetus. It connects via the umbilical cord to the baby.

The fetus is enclosed in a fluid-filled sac surrounded by a thin membrane, called the amnion. A second membrane, called the chorion, surrounds the amniotic sac. Let’s break it down:

  • Mo: Means one
  • Di: Means two
  • Chorion: Is the outer membrane 
  • Amnion: Is the inner membrane, which forms a sac filled with amniotic fluid containing the fetus

Mo Twin Terms

These terms describe twins—always monozygotic twins—that share an environment. They are enclosed in ONE amniotic sac and/or ONE chorion:

  • Monochorionic: One chorion
  • Monoamniotic(or monoamnionic): One amnion

Di Twin Terms

These terms describe twins—which can be monozygotic or dizygotic—that each have a separate sac. There are two placentas, although in some cases, the placentas fuse together into one:

  • Dichorionic: Two separate chorions
  • Diamniotic: Two amnion/two amniotic sacs


So let’s look at them in combination. The majority of twins are di/di, that is, dichorionic, diamniotic. That means that they developed separately, each with their own placenta and own sacs.

This is how all dizygotic, or fraternal twins develop. In addition, some monozygotic (identical) twins will also form this way, depending on how early after conception the egg splits.

However, some monozygotic twins will be mo/di or mo/mo. That means they are monochorionic but diamniotic, or they are monochorionic and monoamniotic. Twins cannot be di/mo.

If there are two chorions, then they can not be in the same amniotic sac since it is the innermost membrane and, by definition, they will be separated. Let’s break it down further.

  • Di/Di (dichorionic/diamniotic): These are twins that are separate, with separate chorions and in their own amniotic sac. (Sometimes designated as DCDA.)
  • Mo/Di (monochorionic/diamniotic): These are twins that are in separate amniotic sacs but contained within the same outer membrane. There is a single, shared placenta. (Sometimes designated as MCDA.)
  • Mo/Mo (monochorionic/monoamniotic): These are twins that are in a single amniotic sac, with a single chorion. There is a single, shared placenta. (Sometimes designated as MCMA.)

Think of It This Way

  • Di/Di twins are like two houses in the same neighborhood. Each baby lives in his own house and manages his own resources.
  • Mo/Di twins are like two apartments in the same building. Each baby lives in the same building, but they have their own private space. They are separated by interior walls and doors, but the exterior of the building is shared.
  • Mo/Mo twins are like two individuals living in the same room in the same house or apartment. They share the resources that come into the space within the same walls.

How Is Twin Chorionicity Identified?

During pregnancy, an examination of the placenta(s) and membranes via ultrasound can sometimes confirm chorionicity. An ultrasound scan, usually early in the pregnancy, will check for one or two placentas and the thickness of membranes.

A thicker membrane can indicate di/di twins, while a thin membrane or the absence of a membrane can signal mo/di or mo/mo twins. Sonographers can also check for the “twin peak sign," also called the “lambda sign” because of its resemblance to the Greek letter λ. It forms where the two placentas and sacs abut in di/di twins. A T-shaped structure can indicate mo/di twins.

The only definitive confirmation of di/di twins during pregnancy, however, is when the babies are identified as one boy and one girl. If they are different sexes, they are dizygotic, with rare exceptions. Remember, all dizygotic twins are di/di, while monozygotic twins can be di/di, mo/di or mo/mo.

After delivery, a pathological analysis of the placenta and membranes can give some insight. But it’s most important to determine chorionicity during pregnancy in order to monitor and address any potential complications.

What Are the Ramifications?

Knowing the chorionicity of twins can help assess risk and determine a treatment plan to provide proper prenatal care for an optimal outcome of the twin pregnancy. Dichorionic/diamniotic (di/di) twins generally present fewer additional risks.

However, mo/di and mo/mo twins should be monitored carefully because they are at increased risk for complications. Twin-to-twin transfusion syndrome (TTTS) or twin anemia-polycythemia sequence (TAPS) can develop in mo/di twins. Mo/mo twins can be compromised by cord entanglement.

Unique and Unusual Types of Twins

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Sonography of Twins

Ultrasound surveillance in twin pregnancy: An update for practitioners

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Now discussing:

Monochorionic twins

Identical twins that share the same placenta

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides.

Monochorionic twins are monozygotic (identical) twins that share the same placenta. If the placenta is shared by more than two twins (see multiple birth), these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies.[1] Seventy-five percent of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic.[2] If the placenta divides, this takes place before the third day after fertilization.[2]

Amniocity and zygosity[edit]

Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), but sometimes, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization.[2] Monoamniotic twins are alwaysmonozygotic (identical twins).[3] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.[2]


λ Sign in 8 week pregnancy

By performing an obstetric ultrasound at a gestational age of 10–14 weeks, monochorionic-diamniotic twins are discerned from dichorionic twins. The presence of a "T-sign" at the inter-twin membrane-placental junction is indicative of monochorionic-diamniotic twins (that is, the junction between the inter-twin membrane and the external rim forms a right angle), whereas dichorionic twins present with a "lambda (λ) sign" (that is, the chorion forms a wedge-shaped protrusion into the inter-twin space, creating a rather curved junction).[4] The "lambda sign" is also called the "twin peak sign". At ultrasound at a gestational age of 16–20 weeks, the "lambda sign" is indicative of dichorionicity but its absence does not exclude it.[5]

In contrast, the placentas may be overlapping for dichorionic twins, making it hard to distinguish them, making it difficult to discern mono- or dichorionic twins on solely the appearance of the placentas on ultrasound.


In addition to a shared placenta, monochorionic twins also have their circulatory systems intermingled in random and unpredictable circulatory anastomoses. This can cause disproportionate blood supply, resulting in twin-to-twin transfusion syndrome (TTTS) in 20%[1] of MoDi pregnancies. This is the main complication of monochorionic twins.

The 80% of MoDi pregnancies without TTTS still have high rates of birth weight discordance, fetal growth restriction, prematurity and resultant cesarean section deliveries.[1] One twin may also fail to develop a proper heart and become dependent on the pumping activity of the other twin's heart, resulting in twin reversed arterial perfusion.[2] If one twin dies in utero, blood accumulates in that twin's body, causing exsanguination of the remaining twin.[2]

In the case of monoamniotic twins the risk of complications is substantially higher because of additional potential umbilical cord entanglement and compression.[3] However, the perinatal mortality of monochorionic twins is fairly low.[1]

See also[edit]


  1. ^ abcdCordero L, Franco A, Joy SD, O'shaughnessy RW (December 2005). "Monochorionic diamniotic infants without twin-to-twin transfusion syndrome". J Perinatol. 25 (12): 753–8. doi:10.1038/ PMID 16281049.
  2. ^ abcdefShulman, Lee S.; Vugt, John M. G. van (2006). Prenatal medicine. Washington, D.C.: Taylor & Francis. p. 447. ISBN .
  3. ^ abPregnancy-Info -- > Monoamniotic Twins Retrieved on July 9, 2009
  4. ^Sepulveda, W.; Sebire, N. J.; Hughes, K.; Odibo, A.; Nicolaides, K. H. (1996). "The lambda sign at 10-14 weeks of gestation as a predictor of chorionicity in twin pregnancies". Ultrasound in Obstetrics and Gynecology. 7 (6): 421–423. doi:10.1046/j.1469-0705.1996.07060421.x. PMID 8807758.
  5. ^Sepulveda, W.; Sebire, N.; Hughes, K.; Kalogeropoulos, A.; Nicolaides, K. (1997). "Evolution of the lambda or twin-chorionic peak sign in dichorionic twin pregnancies". Obstetrics & Gynecology. 89 (3): 439–441. doi:10.1016/S0029-7844(97)85858-9. PMID 9052601.

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