Twins can be described in different ways. Maybe you have heard parents say their kids are "mirror twins", or you see twin girls dressed alike and assume that they are identical twins. These terms sound alike but have some key differences.

Here's what you should know about identical twins and mirror twins, and how the terms are related.

Identical Twins

Identical twins are more than just twins that have a similar appearance. The term actually describes how twins form and refer to their zygosity.

Identical twins are actually monozygotic twins. When a single, fertilized egg splits into two and develops into two embryos, the twin babies are monozygotic.

The twins develop from the same fertilized egg, so they have the same genetic origins. They often have similar physical characteristics and may even look exactly alike—which is why they are sometimes called “identical” twins.

Identical twins aren’t exactly alike in every way, because humans are influenced by more than their genes. Environmental influences also determine how an individual looks and behaves.

Mirror Twins

Mirror twins, or mirror image twins, are not a real category of twins as identical and monozygotic twins are. The term “mirror twins” describes a characteristic of some twins where their features appear asymmetrically—that is, on opposite sides.

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

The shape or placement of each twin's facial features, such as eyebrows, nostrils, or ears, also appears opposite. The visual effect is such that when they are facing each other, the twins seem to be reflections—as if looking in a mirror.

Potential Physical Characteristics of Mirror Twins Birthmarks

Moles

Freckles

Dimples

Eye shape

Eyebrow shape

Nostril shape

Ear shape

Cowlicks

Hair whorls

Teeth

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. Handedness isn’t necessarily determined by genetics).

One twin may prefer to sleep on the left side while the other prefers the right. Mirror twins may display different characteristics because of brain hemisphere dominance.

It has been theorized that the different hemispheres of the brain control different thought processes. For example, if an individual is more dominant on the left side of the brain than the other, they may be stronger in skills that require logic or analysis rather than intuition or creativity.

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.﻿﻿

What Causes Mirror Twinning?

If mirror twins are a consequence of monozygotic twinning, it’s been theorized that mirror image twinning occurs when the egg splits later on. Generally, more than a week after conception, but not late enough 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.

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

In the case of situs inversus, X-ray, CT scan, MRI, or ultrasound assessment can be used to identify the position of internal organs.