How Monozygotic Twins Develop

Identical twins facing each other
Monozygotic twins are identical twins. Image Source / Getty Images

Monozygotic twins are formed when one zygote, created with one egg and one sperm, splits into two. Instead of having just one embryo—which is usually what you get from one egg and one sperm—the result is two embryos. Each of those embryos develops as a separate fetus.

Monozygotic twins are also known as identical twins because they practically share identical genes. However, they are not exactly genetically identical. While we once thought mono-twins were genetically identical, with the science we have today, the name may no longer be accurate.

Monozygotic twins are the same sex (except in some extremely rare situations, more below), share physical characteristics, and even may have similar personalities.

What Causes Monozygotic Twins to Occur?

No one really knows what causes monozygotic twins. This is an area of ongoing research.

Identical twins do not (generally) run in families. This implies that identical twins form when something in the environment triggers the split, or possibly, they occur randomly.

(There are some isolated populations where identical twins do seem more likely to occur, and there may be a genetic connection. But this is rare, and a gene has yet to be identified that increases the risk of monozygotic twinning.)

IVF research has given us some insight into how identical twins may form. IVF embryos are more likely than naturally conceived embryos to split into identical twins. A fertility doctor can transfer just one embryo—in hopes of reducing the risk of non-identical twins—but identical twins may still occur, and more frequently than in the general population.

To investigate, researchers set cameras to take photos every two minutes of embryo development. In normal embryo development, a fluid-filled cavity grows inside the embryo. This is known as the blastocoel.

Also inside the embryo is a collection of cells known as the inner cell mass. This collection of cells will eventually form the fetus. In some cases, the blastocoel collapses on itself. This typically results in the destruction of the embryo. However, sometimes, the embryo survives the collapse.

What seems to happen in these cases is the embryo collapses in on itself, causing the inner cell mass to split into two. The two inner cell masses lead to the development of twins.

It’s important to note that we can’t really know if this is how monozygotic twins develop in the womb. But it gives us insight into a process that has, until recently, been a complete mystery.

But why might this happen more frequently with IVF?

For one, an embryo is kept in an artificial solution in the lab. While scientists have done their best to make it as close to natural as possible, it is still not the same environment an embryo would develop inside a woman’s reproductive system. This solution might increase the risk of the collapse.

Secondly, there are differing opinions on when to transfer the embryo into the woman’s uterus. It seems that transferring the embryo later may increase slightly the odds of identical twinning.

The Genetics of Twinning

It was once thought that monozygotic twins shared the exact same DNA. But this isn’t true.

For one, every time cells split, there is a risk of mutations occurring. These mutations can occur as early as the first split. This is one reason why identical twins are at an increased risk of congenital diseases.

After that very first split, the individual cell masses continue splitting on their own. With every split, there is a risk of a mutation.

At birth, the genetics of twins are extremely, extremely similar—but they are not identical. Over time, their genetic similarities continue to decrease. This is due to epigenetics—the science of how the environment changes our DNA.

Some studies have found that slight DNA differences are more common in older identical twins than in very young identical twins. This is likely due to spending more time apart and therefore being exposed to different environments.

Identical Twins…But Different Genders?

While extremely rare, it is possible for monozygotic twins to be two different genders. This goes back to the idea that genetic mutations can occur from the very first split.

If an egg carries double-X chromosomes (when a normal egg should carry just one X-chromosome) and is fertilized with a Y-sperm, you can get an XXY embryo. This is also known as Klinefelter syndrome.

However, what happens if that XXY embryo splits into monozygotic twins? You can end up with one twin with XX expression (female), and the other with XY (male.) How rare is this situation? It has only been described in medical literature four times.

There is another (rare) way you can have identical twins of different genetic genders.

If you have an egg with one X chromosome (as it should), and sperm with a Y-chromosome, you would typically get a boy (XY). Normally, if this embryo split into twins, you’d get identical twin boys. However, it’s possible for one twin to result with only the X chromosome (usually written as XO), and the other XY.

Thus, one twin will be a girl (with the congenital disorder known as Turners Syndrome), and the other twin will be a boy. All this said, these situations are so rare, you can assume that 99.999% of boy-girl twins are not monozygotic twins.

Does a Family History of Identical Twins Increase the Odds of Having Them?

Contrary to popular belief, your chances of having monozygotic twins are not related to your family history.

If there is more than one set of identical twins in a family, it is more likely due to luck or external environmental factors—but not family genetic history.

There are some tribal and isolated populations where twins seem to run in the family. (Or in the greater tribe, for that matter.) It remains unknown if genetics are at play here or the environment.

Dizygotic twins—non-identical twins—do however run in families. Fertility drugs can also increase your risk of having twins.​

Semi-Identical Twins and Conjoined Twins

A rare form of monozygotic twins is semi-identical or half-twins. This occurs when two separate sperm fertilizes one egg. (This is another situation where you can get two genders, but these are not truly “identical” twins since you started with two and not one sperm fertilizing the egg.)

Conjoined twins are another rare form of monozygotic twinning, where the twins do not fully separate when the zygote splits. They may share multiple organs. Most conjoined twins die in utero or are stillborn.

In some cases, conjoined twins who survive may be surgically separated. This surgery is risky and cannot always be attempted or completed successfully.

Monozygotic Twins, Their Amniotic Sacs, and Placentas

Most of the time, monozygotic twins have separate amniotic sacs but share one placenta. The technical term for this is monochorionic-diamniotic (or Mo-Di), and it occurs between 60 and 70 percent of the time with monozygotic twins.

Sharing one placenta increases risks to the pregnancy, due to the possibility of twin-to-twin transfusion syndrome. The pregnancy must be watched very carefully if Mo-Di twins are diagnosed.

Another possibility is for the twins to each have his or her own placenta and amniotic sac. This is known as dichorionic-diamniotic (or Di-Di) twins. The risks in a Di-Di pregnancy are lower than with a Mo-Di pregnancy. There is a misconception that Di-Di twins are always fraternal (non-identical), but this is not true. About 30% of monozygotic twins are Di-Di.

The riskiest combination is when twins share one amniotic sac and one placenta. This only occurs in monozygotic twins and never with non-identical twins.

This is known as monochorionic-monoamniotic (Mo-Mo) twins, and it is the relatively rare, occurring in only 5% of twin pregnancies. With Mo-Mo twins, the biggest risk is that the babies can become entangled in the umbilical cords. There is also a risk of twin-to-twin transfusion syndrome and a higher risk of prematurity. Early studies found that only 50% of Mo-Mo twins survived, but later studies have found more encouraging results, with the perinatal (the period immediately before and after birth) mortality rate closer to 20%.

How Will You Know If Your Twins Are Identical or Not?

The only way to know for sure if your twins are identical is through a DNA test. There are clues and possible signs twins might be identical (or not identical), but DNA testing is the only way to know for certain. You can talk to your pediatrician about DNA testing, or there are a number of genetic testing companies that will do the testing for you.

Your twins are probably not monozygotic/identical if...

  • They are boy/girl twins
  • They have different blood types
  • They look very different

Your twins might be monozygotic/identical if...

  • They share one placenta
  • They are the same sex and look very similar

Your twins are monozygotic/identical if...

  • They share one amniotic sac
  • DNA testing confirms they are identical
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