If you or someone you know is expecting twins, you may have a lot of questions. Twins are becoming more common, accounting for about 3.3 out of every 100 births in the US, which is up from 1.9/100 births in 1980. The increase is largely due to the use of assisted reproductive technologies. With all these twins, you may have heard of “mirror twins.” How do they differ from identical twins?
Some Basic Twin Terminology and Physiology
Most people know that there are two basic kinds of twins: fraternal twins and identical twins. Fraternal twins occur when two different sperm fertilize two different eggs. These twins are no different than any other set of siblings: they can be the same gender or different genders, and can look alike (or not alike) as any other pair of non-twin siblings.
Identical twins occur when one fertilized egg splits into two, resulting in twins that have the exact same DNA. A fertilized egg is also called a zygote, so identical twins are monozygotic (coming from one zygote), and fraternal twins are called dizygotic (coming from two zygotes).
But it gets more complicated that that. Fraternal twins always have their own placenta (chorion) and their own amniotic sac and so they are dichorionic and diamniotic. Identical twins can share both a placenta and a sac (monochorionic and monoamniotic), share a placenta but have their own sacs (monochorionic and diamniotic), or have their own placentas and sacs (dichorionic and diamniotic). These terms are used here mostly as a reference tool for you when reading about twins.
Identical twins, as the name implies, often do look a lot alike. But so do lots of non-twin siblings. Therefore, some assumptions are made when twins are born: opposite-sex twins mean they are fraternal, same-sex twins sharing one placenta (monochorionic) means they are clearly identical, but same-sex twins with two placentas (dichorionic) can be either fraternal or identical. (It is possible, though extremely rare, for identical twins to be different genders, as explained here.)
But What Are Mirror Twins?
Mirror twins (or mirror-image twins) are identical twins that have some physical features that are asymmetrical. For example, one twin has a hair whorl on the back of his head going clockwise, while the other twin has his going counter-clockwise. Or one twin has a mole on the left cheek, while the other has one on the right. One theory states that if the zygote splits at day 9-12, then some mirror imaging occurs. As many as 25% of identical twins have physical features that are mirror images.
The mirror imaging is most evident when physical defects or malformations are found. The scientific literature has dozens of reports of various anomalies that are mirror images in identical twins, including cleft lip and palate, eye and ear defects, bony abnormalities, optic gliomas, and arachnoid cysts (benign cysts in the brain).
In some cases, even internal organs are positioned in mirror images in twins. In one case report, one twin had the heart on the right side and the stomach on the left (normal), and the other twin had the heart on the left side and the stomach on the right.
Some have used the idea of mirror twinning to explain why some identical twins have different dominant hands. Although left-handedness is more common in twins, several studies have not shown that different handedness in twins is due to mirror imaging.
Interestingly, one pair of fictional twins were created to demonstrate mirror twinning. According to one scholar, Lewis Carroll wrote Tweedledee and Tweedledum in Through the Looking Glass to be perfect mirror images of each other, including in handedness, physical features, and thought.
On a personal note, I will admit that I am the father of identical twin girls who have some features that are mirror images, though they are minor. As babies, the mirror image features helped us identify which one was which if my wife or I got confused. And if you are expecting twins, let me reassure you that it is truly a blessing. Being a parent is always a wonderful experience, but it’s a special privilege to raise twins. It’s twice the work, but double the fun.