If you or somebody is expecting twins, you may have lots of questions. Twins have become more prevalent, 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 basically because of the use of assisted reproductive technologies. With all of these twins, you may have heard about “mirror twins.” How do they vary from identical twins?
Some Basic Twin Terminology and Physiology
Most people realize that there's two basic types of twins: fraternal twins and identical twins. Fraternal twins occur when two different sperm fertilize two different eggs. These twins are no diverse from every other set of siblings: they may be the same gender or different genders, and may look alike (or otherwise alike) as any other pair of non-twin siblings.
Identical twins occur when one fertilized egg splits into two, leading to twins that have the very same DNA. A fertilized egg can also be called a zygote, so identical twins are monozygotic (coming from one zygote), and fraternal twins are known as dizygotic (coming from two zygotes).
But it gets more complicated that that. Fraternal twins also have their very own placenta (chorion) as well as their own amniotic sac and they also are dichorionic and diamniotic. Identical twins can share both a placenta and a sac (monochorionic and monoamniotic), share a placenta but their very own sacs (monochorionic and diamniotic), or have their own placentas and sacs (dichorionic and diamniotic). These terms are utilized here mostly as a reference tool for you personally when reading about twins.
Identical twins, as the name implies, often do look a lot alike. But so plenty of non-twin siblings. Therefore, some assumptions are created when twins are born: opposite-sex twins mean they are fraternal, same-sex twins sharing one placenta (monochorionic) ensures they are clearly identical, but same-sex twins with two placentas (dichorionic) could be either fraternal or identical. (It's possible, though extremely rare, for identical twins to become different genders, as explained here.)
But What Are Mirror Twins?
Mirror twins (or mirror-image twins) are similar twins that have some physical features which are asymmetrical. For instance, one twin includes a hair whorl on the back of his head going clockwise, while the other twin has his going counter-clockwise. A treadmill twin includes a mole on the left cheek, while the other has one around 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 which are mirror images.
The mirror imaging is best shown when physical defects or malformations are found. The scientific literature has dozens of reports of various anomalies which are mirror images in identical twins, including cleft lip and palate, eye and ear defects, bony abnormalities, optic gliomas, and arachnoid cysts (benign cysts within the brain).
In certain cases, even internal organs are situated in mirror images in twins. Once report, one twin had the center around the right side and also the stomach around the left (normal), and also the other twin had the center around the left side and also the stomach on the right.
Some have used the idea of mirror twinning to describe why some identical twins have different dominant hands. Although left-handedness is more common in twins, several research has not shown that different handedness in twins is due to mirror imaging.
Interestingly, one pair of fictional twins were created to show mirror twinning. Based on one scholar, Lewis Carroll wrote Tweedledee and Tweedledum in Through the Looking Glass to become perfect mirror images of each other, including in handedness, physical features, and thought.
On an individual note, I'll admit that i'm the daddy of identical twin girls who've some features that are mirror images, though they are minor. As babies, the mirror image features helped us identify which was which if my spouse or I acquired confused. And if you are expecting twins, allow me to reassure you that it's truly a blessing. Motherhood is always an excellent experience, but it’s a unique privilege to boost twins. It’s twice the work, but double the amount fun.