Mirror Twins Facts: Learn What They Are

Mirror twins simply mean identical twins.

Fraternal (or dizygotic) twins are the most common type of twins and share about 50% of their DNA. Whereas identical twins share 100% of their DNA and are also called monozygotic twins, their physical appearance is just a mirror image of each other.

It is often thought that twins that are identical are the same as mirror-image twins. But that’s not true. In a set of mirror twins, features appear asymmetrically, that is, on opposite sides. It is like looking in the mirror when they face each other. They do have defining characteristics such as dominant hands, birthmarks, dental structure, organs (situs inversus), or other traits on opposite sides. When a fertilized egg splits in the embryonic stage, around days 9–12, the pregnancy results in a mirror twin set.

As monozygotic twins share the same genetic material, they can be detected with a DNA test, but in the case of mirror twins, this is not possible. Moreover, a rare twin type in which the two siblings are physically connected is known as a conjoined twin. They share an amniotic sac, chorion, and placenta. This happens when an initial embryo only partially splits into two separate embryos.

Did you know twins start interacting in the womb at about 14 weeks? Moreover, several studies have shown that separating twins from each other can have negative physical and mental health consequences for both children. Some studies also suggest that mothers of twins tend to live longer. Currently, Nigeria has the highest rate of multiple births and the most identical twins, while China has the lowest.

Mirror Twins Meaning

Twins share a special bond. The term 'mirror twin' doesn't refer to a scientific category but a phenomenon. Mirror twins are identical but with asymmetrical physical appearances. They aren’t a real category of twins but an offshoot of identical twins. These types of twins are like mirror images as defining characteristics such as dental structure, birthmarks, dominant hands, or different features are on opposite sides.

In layman's terminology, a mirror image twin set forms when a single fertilized egg splits to create two babies. These babies share the same genetic makeup as they come from a single egg. All mirror image twins are identical twins, but not all identical twins are mirror twins. Mirror twins often self-identify as they have opposite anatomical traits, such as one having a mole on the right cheek and the other having it on the left side. They also have identical eyes, skin, and hair color. In fact, their weight, size, hairstyles, fashion styles, and even their mannerisms are almost mirror images.

Mirror image twins can occur among identical twin sets about 25% of the time. In some cases, they are also called 'messy group twin sets' because they share common features like headaches, tension, biting fingernails, and whatnot. However, mirror imaging is never the same for every set of twins, and differences are exhibited and increase as the twins age.

How are mirror twins formed?

Mirror twinning most often occurs due to the fertilized egg splitting or dividing later than normal. The separation takes place between 7-12 days after fertilization by a single sperm, and the identical halves evolve into separate babies and become genetically identical. Scientists say that the fertilized egg is more likely to split between two and eight days after conception to form a mirror-image twin set.

It has also been theorized that the genes activated by the egg determine the right and left sides of the embryo. Moreover, these twin sets have a higher risk correlated with certain medical conditions. However, many mirror image twins display aspects of late-separating monozygotic twins and are also monochorionic twins, who in fact share the same placenta, or monoamniotic twins, who share an amniotic sac in the womb itself.

A very fascinating fact here is that there is quite a possibility for twins to have different dads.

Characteristics Of Mirror Twins

Mirror image twins are way more interesting than thought. When one twin has the same features as the other but on the other side, they are referred to as mirror twins. They also have functional mirroring with asymmetrical physiological features. Functional mirroring exhibits differences such as dental features and mirrored internal organs, whereas physiological mirroring shows opposite temperaments, sexual orientation, and even career interests.

However, some of the physical characteristics of mirror-image twins include birthmarks and moles, which appear asymmetrically, and hand orientation, such as if one twin is left-handed, then the other is right-handed. Eye shape and eyebrow shape could be another feature that is also found reversed in these twin sets.

Freckles and dimples, nostril shape, and ear shape are also asymmetrical, and hair whorls are in opposite directions too. What’s more interesting is that these twins are not only reverse asymmetric in appearance but also with internal organs. In extremely rare cases, the position of the internal organs is on the opposite side, which is very rare. It is referred to as ‘situs inversus', meaning organs of the body like the stomach, heart, or liver are on the opposite side of their normal body position.

There are twins who feel each other’s pain. However, mirror twins feel pain psychologically and with empathy due to mirrored internal organs. Furthermore, mirror-imaged twins have opposite personalities, such as opposite sleeping sides, behavioral orientation, and altered gaits; for example, one twin may be very introverted while the other twin may be an extrovert.

Difference Between Mirror Twins And Twins

Observing physical features is the basic way to determine whether twins are mirror twins or not. However, DNA testing confirms if twins are similar enough to be classed as monozygotic or dissimilar enough to be dizygotic.

According to research, out of four identical twins, one will be a mirror image, and out of every 1,000 people, 32 will be twins. But the question arises: are mirror twins the same as identical twins or other types of twins? The answer to this is, No. Identical twins have few visible differences in physical features, but mirror twins are carbon copies of each other but reversely. Unlike fraternal twins, mirror twinning has a greater possibility of getting the same illness.

Considering development, two different sperm and two separate eggs create two babies in non-identical twins, but in mirror image twins, only one egg splits up into two. These twin sets always have the same gender. Likewise, mirror twins have the same blood type. Mirror twins are a fascinating type of twin set. They share the same genetic makeup. However, their fingerprints do not match. In rare cases, fingerprints might be similar, but with the opposite pattern. Similarly, twins' cowlicks and birthmarks may match each other.