And while most of us know a pair of fraternal or identical twins — after all, if you grew up in the 80s or 90s, you probably watched a well-known set on “Full House” — fewer know about mirror twins.
The phenomena — which occurs in approximately 25 percent of all identical twins, according to Laura Segal, PhD, director of the Twin Studies Center at California State University, Fullerton — is common but relatively unknown.
So how do you know if you have (or are a) mirror twin? Here’s everything you need to know about this fascinating condition.
The term mirror twin is used to describe a type of identical, or monozygotic, twin pairing in which the twins are matched as if they’re looking into a mirror — with defining characteristics like birthmarks, dominant hands, or other features on opposite sides.
Mirror twins always come from a single fertilized egg and share the same genetic makeup. That said, the exact cause of the mirroring in some twin pairs is unclear. Some researchers suggest mirror twins occur when an egg splits later in the fertilization process.
Eggs usually split within a week of conception (or less). However, some split later, around 7 to 10 days after — and this may explain the variation. However, researchers have yet to determine why mirroring occurs.
While all mirror twins are identical, not all identical twins are mirror twins. After all, mirror twins only occur when a single egg splits and that split causes specific traits to develop.
According to Leann Poston, MD, a medical content expert at Invigor Medical, “Mirror twins are a type of identical twins which occur when an egg splits later in development and when said egg has already activated the genes that determine the right and left side of the embryo.”
While mirror twins often self-identify because they have opposite anatomical features (one has a freckle on the left cheek, while the other has one on the right cheek), they do typically have identical skin, eye, and hair colors. Their size and weight may also be similar.
There is no specific test used to diagnose mirror twins. A DNA test can confirm whether twins are identical, but there are no markers related to mirror twins.
Rather, the condition is self-identified. When twins have opposite features from their sibling, they may refer to themselves as mirror twins.
Some examples of mirrored features include:
Mirror twins may also have asymmetrical physiological features, with “functional mirroring.” Some mirror twins, for example, will sleep on opposite sides while others have altered gaits — one leads with their right foot, while the other leads with their left.
Poston tells Healthline “one mirror twin may be left-handed and the other right-handed, and fingerprints, while not identical, may be similar and present in an opposite pattern.”
One older study from 1993 noted that some mirror twins exhibit “medical mirroring” — such as dental features as described in this Scientific American blog post — or “psychological mirroring” — opposite career interests, temperaments, and even sexual orientation.
In some rare cases, mirror twins have a condition known as situs inversus, which causes their internal organs to be arranged on opposite sides of their body. The liver on one twin, for example, would be in its usual place — in the upper right side of the abdomen — while the liver in the other twin would be on the left.
An X-ray, CAT scan, MRI, or ultrasound can be used to diagnose this condition.
That said, it is important to reiterate that mirror twinning — in and of itself — isn’t an official diagnosis. Rather, mirror twins are a subset of identical twins which describes an aspect of their physical appearance and/or movements.
While the term mirror twin may not be well-known, the phenomena is. In fact, millions of identical twins can and do identify as “mirror twins.” The best way to determine if you — or your littles — are mirror twins is to look (yes) in the mirror.