Difference Between Mirror and Identical Twins

You may have heard twins described in different ways. Perhaps you have heard parents say that their children are mirror twins. Or you see a set of twin girls dressed alike and assume that they are identical twins. But do you really understand these terms? Find out more about identical twins and mirror twins, and how they are related. 

Identical twins are more than just twins that have a similar appearance. The term identical twins actually describes how twins form and refer to their zygosity. Identical twins can be more accurately called monozygotic twins. Monozygotic twins form when a single, fertilized egg splits into two and develops into two embryos, producing two babies. Since the two individuals develop from the same fertilized egg, they have the same genetic origins and may have very similar physical characteristics. They may even look exactly alike. Thus, they are known as “identical” twins. 

Of course, identical twins aren’t exactly alike in every way, because human beings are influenced by more than their genes. Environmental influences also impact how a person looks and behaves. 

Mirror twins—or mirror image twins—aren’t really a category of twins, like identical/monozygotic twins. Rather, the term “mirror twins” describes a characteristic of some twins, where their features appear asymmetrically -- that is, on opposite sides. A birthmark may manifest on the left side of one twin but on the right side of the other. Facial features such as dimples may be on opposite sides of the face. The shape or placement of facial features such as eyebrows, nostrils or ears appear opposite so that when facing each other, the twins seem to be reflections as if looking into a mirror. Cowlicks, or whorls in the hair, may run clockwise on one twin, and counterclockwise on the other.

Here are some examples of physical characteristics that may indicate mirror twins. 

• Birthmarks
• Moles
• Freckles
• Dimples
• Eye shape
• Eyebrow shape
• Nostril shape
• Ear shape
• Cowlicks
• Hair whorls
• Teeth

Gestures or movements may also be manifestations of mirror image twinning. One twin may be right-handed and the other left-handed, although many twins, regardless of zygosity share this characteristic as 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 due to brain hemisphere dominance. It is theorized that the different hemispheres of the brain control thought processes, so if an individual is more dominant on one side of the brain, 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, where internal organs such as the heart, liver, lungs or stomach are situated on the opposite side of their normal anatomical position.   

If mirror twins are a consequence of monozygotic twinning, it’s theorized that mirror image twinning occurs when the egg splits later, nine to twelve days post conception, but not late enough that conjoined twins form. Thus, many mirror image twins also display characteristics of other late-splitting monozygotic twins and are also monochorionic or monoamniotic in the womb. 

There’s no test to confirm mirror twinning; DNA analysis will only confirm that the twins’ genetic characteristics are similar enough to be considered monozygotic. Observation of their physical features is really the only way to assess and determine mirror twins. In the case of situs inversus, an assessment using X-ray, CT scan, MRI or ultrasound can identify the position of internal organs.