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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.

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 65 percent of identical twin pregnancies are monochorionic (one chorion). The other 35 percent 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.

Mono/di twins occur in about 1 in 3 instances 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.

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!