What Are Monozygotic Twins?

Monozygotic twins are also known as identical twins. Here's why they develop, how common they are, and what to expect during pregnancy.

Twin girls sit on a sofa

Kike Arnaiz / Stocksy

Have you heard the term "monozygotic twins?" It may sound super scientific, but it's actually another way to describe identical twins.

A monozygotic twin pregnancy starts as one fertilized egg initially, but it splits into two completely separate embryos shortly after conception. Monozygotic twins can share a placenta and an amniotic sac, or they can each have their own.

Keep reading to learn more about how monozygotic twins form, how common they are, and how they differ from dizygotic (fraternal) twins. We also discuss what to expect during a pregnancy with monozygotic twins.

What Are Monozygotic Twins?

Monozygotic twins result from single egg fertilized by a single sperm. During the early stages of conception, it divides into two separate embryos.

How Common Are Monozygotic Twins?

Monozygotic twins are relatively uncommon, occurring naturally in around three to five out of every 1,000 births around the world. About one-third of all twins are classified as identical.

Some factors can slightly increase your chances of having monozygotic twins. According to one study, using fertility treatments like in vitro fertilization (IVF) can raise your chances of having identical twins when compared to natural conception.

Alex Robles, a board-certified OB-GYN and reproductive endocrinology and infertility specialist at Columbia University Medical College, also points to a small-scale study conducted in Africa, whose findings indicate that a family history of monozygotic twins might increase your likelihood of having them. But it's worth noting that others studies haven't found a genetic component to identical twins.

How Do Monozygotic Twins Develop?

During ovulation, a mature egg is released from the ovary, and it travels down the fallopian tube. If the egg meets sperm in the reproductive tract, fertilization can occur, resulting in conception. Eggs that aren't fertilized by sperm travel shed with the uterine lining during a menstrual period.

Monozygotic twins arise from a single fertilized egg that divides into two embryos during the first few days of development, says Dr. Robles. These two embryos become two babies. "It is not fully understood how or why this happens, but genetics may contribute to the process," he says. This is the main way in which monozygotic (identical) twins differ from fraternal (dizygotic) twins.

What's the Difference Between Monozygotic and Dizygotic Twins?

Monozygotic (identical) and dizygotic (fraternal) twins differ based on the number of fertilized eggs involved in their development, says Dr. Robles. "Monozygotic twins come from a single fertilized egg, while dizygotic twins arise from two different eggs that are fertilized by separate sperm."

Because they originate when one egg splits in two, monozygotic twins are always the same sex, and they're more similar in appearance than dizygotic twins. In fact, dizygotic twins can look and act completely different, just like any two non-twin siblings might.

What's more, monozygotic twins share the same genetic material, while dizygotic twins only share about half, according to the National Human Genome Research Institute.

How Are Dizygotic Twins Different?

Dizygotic (fraternal) twins happen when two different eggs are fertilized by separate sperm.

Types of Monozygotic Twin Pregnancies

There are three different types of monozygotic twin pregnancies: monochorionic-diamniotic (mo-di), dichorionic-diamniotic (di-di), and monochorionic-monoamniotic (mo-mo). They're determined based on whether the babies share a placenta and/or amniotic sac. Here's what to know about each one.

  • Monochorionic-Diamniotic (Mo-Di): These identical twins have separate amniotic sacs but share one placenta. This is the most common type of monozygotic twin pregnancy.
  • Dichorionic-Diamniotic (Di-Di): These identical twins each have their own amniotic sac and their own placenta. This type of identical twin pregnancy is associated with the lowest amount of risk to each baby.
  • Monochorionic-Monoamniotic (Mo-Mo): The rarest and riskiest type of monozygotic twin pregnancy, these identical twins share both a placenta and an amniotic sac. Studies indicate this type of pregnancy is associated with a 70% chance of survival for both twins, and many doctors will insist on delivering via C-section around weeks 32-34. Risks include umbilical cord entanglement and twin-to-twin transfusion syndrome (when one baby gets more blood supply and the other becomes malnourished).

Pregnancy With Monozygotic Twins: What to Expect

If you're expecting identical twins, your pregnancy may be considered higher risk than a singleton pregnancy, no matter your age, physical condition, or number of previous pregnancies. There's simply more to track, and a slight increase in the number of possible complications.

According to March of Dimes, some risks associated with monozygotic pregnancies include the following:

When expecting twins, all of your pregnancy symptoms may be slightly exaggerated. You'll likely feel more tired and nauseous in the first trimester, you might gain a bit more weight overall, and you'll have even more pressure on your bladder in the third trimester (not to mention a harder time trying to get comfortable at night).

You'll likely visit your doctor more regularly with a twin pregnancy, and you may even score a few extra ultrasounds, just to ensure both babies are growing properly. Regular non-stress tests may also be conducted with monozygotic twins. Most pregnancies progress without any complications.

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