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.
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 include: 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. There are also less serious medical conditions that have a higher chance of occurring with mirror twins. These conditions could be benign or require surgery to make them more manageable:
Accessory (extra) toesBone cystsCleft lip and cleft palateEsotropia (eye misalignment)Supernumerary (extra) teethTrigger thumb, or having a thumb locked in a bent position
Facial asymmetry and abnormal dental development may also be observed in mirror image twins.
Identification of 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.
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. 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 (twins who share a placenta) or monoamniotic (twins who share an amniotic sac) in the womb. In other words, these conditions could be indicators of what causes mirror twins.
A Word From Verywell
There is still much to learn about the causes of mirror twinning. Although it can have a larger risk associated with 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. Twins often have special bonds and, as a parent, it can be especially rewarding to see your mirror twins grow and develop in similar, but still individual, ways.
