What Are Mirror Twins?

Twin brothers sitting outside

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What Are Mirror Twins?

Mirror twins, or mirror image twins, is a term used to describe a characteristic of some twins where their features appear asymmetrically—that is, on opposite sides. When these twins are facing each other, it is as if they are looking in a mirror.

You might hear someone call their children "mirror twins", but the words don't refer to a scientific category. Most mirror twins actually fall into the category of identical twins. Specifically categorizing multiples is important when considering zygosity, which is their genetic relationship.

Twins can either be monozygotic or dizygotic.

Identical twins are monozygotic, meaning they came from a single, fertilized egg that split into two and developed into two embryos. Monozygotic twins have the same genetic origins. These twins often have similar physical characteristics and may even look exactly alike. Mirror image twins make up 25% of monozygotic twins. 

Dizygotic, or fraternal, twins occur when multiple eggs are fertilized. They may look completely different or similar like siblings who favor one another.

Characteristics of Mirror Twins

Mirror twins have certain physical characteristics in common. These similarities are just reflected on opposite sides of their bodies.

Physical Traits

Potential characteristics that could look alike in mirror twins are: birthmarks, moles, freckles, dimples, eye or eyebrow shapes, nostril or ear shapes, cowlicks, hair whorls, or teeth.

For example, a similar birthmark may manifest on the left side of one twin and on the right side of the other. Facial features such as dimples may be on opposite sides of the face. Cowlicks may run clockwise on one twin, and counterclockwise on the other.

Gestures or Movements

Gestures or movements can also be manifestations of mirror image twinning. For example, one twin may be right-handed and the other left-handed (although many twins, regardless of zygosity, share this characteristic).

Additionally, one twin may prefer to sleep on their left side while the other prefers the right.

Medical Conditions

In some extreme cases (which are exceptionally rare) mirror twins display situs inversus. In this condition, the internal organs (such as the heart, liver, lungs or stomach) are situated on the opposite side of the normal anatomical position.  

An X-ray, CT scan, MRI, or ultrasound assessment can be used to identify the position of internal organs when situs inversus occurs. This condition is not exclusive to mirror twins, however, and can happen to anyone.

There are also less serious medical conditions that may occur with mirror twins. These conditions could be benign or require surgery to make them more manageable.

  • Supernumerary (extra) teeth
  • Trigger thumb, or having a thumb locked in a bent position
  • Cleft lip/palate
  • Esotropia, also known as eye misalignment
  • Bone cysts
  • Accessory (extra) toes

Facial asymmetry and abnormal dental development may also be observed in mirror image twins.

Identifying Mirror Twins

There is no test to confirm mirror twinning. DNA analysis only confirms that twins’ genetic characteristics are similar enough to be considered monozygotic or dissimilar enough to be dizygotic. Observing physical features is really the main way to determine if twins are mirror twins.

Imaging and diagnostic tests are used to identify other mirror image twinning medical conditions (see section above).

Causes of Mirror Twinning

Because mirror twins may occur with monozygotic twinning, it’s been theorized (and disputed) that mirror image twinning occurs when the egg splits later on during development. Generally, this could happen more than a week after conception, but not late enough so that conjoined twins form.

Many mirror image twins display characteristics of other late-splitting monozygotic twins and are also monochorionic or monoamniotic in the womb. In other words, these conditions could be indicators of what causes mirror twins.

Monochorionic twins are identical twins that share a placenta during pregnancy. Under ideal circumstances, these twins have an equal share of blood flow and nutrients. Complications, requiring treatment, may arise if this is not the case.

Monoamniotic twins are those that share an amniotic sac (where fetuses are contained in waters). This is a rare circumstance, affecting only 1% of twins.

A Word From Verywell

There is still much to learn about the causes of mirror twinning, but there is an abundance of information that we do have about it. Although it can lead to certain medical conditions, most of these scenarios are not cause for major concern.

In fact, being a mirror twin can be a source of great joy for the siblings who experience it. Twins usually have special bonds and a relationship that only they can truly understand. As a parent, it can be especially rewarding to see your mirror twins grow and develop in similar, but still individual, ways.

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Article Sources
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