Unique Types of Twins
Most twins fall into two main categories: identical or fraternal.
Identical twins are monozygotic, meaning that they originate from the same, single fertilized egg. In these cases, the egg eventually splits into two separate parts. But, since both twins still come from the same egg and sperm, they will have completely identical chromosomes.
On the other hand, fraternal twins are dizygotic, so their formation involves two distinct eggs that are released at the same time. Each egg will also be fertilized by a different sperm, so only about half of their chromosomes will be the same. As a result, while fraternal twins might have some shared characteristics, they will not be identical.
But beyond classifying twins as simply identical or fraternal, there are other types of twins that can also occur in unusual pregnancies.
Mirror image twins
Mirror image twins are similar to identical twins, except that they appear as mirror images of each other. The condition causes asymmetrical resemblances so that each twin’s features are opposite that of their sibling.
For example, some mirrored features can include birthmarks, freckles, or dimples on the opposite side of each twin. Mirror twins can also have opposite physiological mannerisms, like right handedness or left handedness.
In extreme cases, a mirror twin can be born with “situs inversus.” This results in the internal organs being placed in the wrong side of the body. For example, one twin might have their liver on the left side of their body instead of the right as it should be. However, the other twin’s liver would be in the normal location. Situs inversus can also cause abnormalities in the arrangement of the heart, lung, and the brain.
In these cases, situs inversus can be diagnosed using an X-ray, MRI, or other type of scan. But, for the most part, twins without situs inversus do not have a standard test to determine if they are mirror twins.
Instead, twins can self-identify as a mirror twin if they observe asymmetrical traits in a each other’s physical appearance or body movements.
Compared with a typical set of identical monozygotic twins, the egg shared by mirror image twins splits much later, more than a week after fertilization has occurred. At this point, the developing embryo already has a distinct left and right orientation, ultimately leading to the asymmetrical features we see once the twins begin to grow up.
Conjoined twins are another variation of identical twins. The clear difference here is that the twins are physically connected.
Conjoined twins are formed when the shared egg develops into an early embryo, but it does not fully split. Two fetuses will develop from this single embryo but remain joined together at a particular part of the body. This area is generally the chest or abdomen. Some conjoined twins may also share internal organs.
Another way conjoined twins can form is if the egg separates into two separate embryos and they partially fuse together again in early stages of development.
Unfortunately, many conjoined twins are stillborn or die shortly after their birth. This is especially common when twins share one or more of their vital organs. As a result, if one twin dies before the other, the other twin will usually follow suit unless swift action is taken.
For instance, if one twin’s heart stops, they will lose blood into the other remaining twin. In the event that surgery is not readily available, the living twin would fall victim to sepsis, a condition that leads to organ failure, shock, and eventually death.
When possible, separation surgery can be a helpful option for conjoined twins who want to avoid such medical complications or have their own independent bodies.
Parasitic twins are a special conjoined twin type in which one twin is underdeveloped and consequently dependent on the other dominant twin for survival.
The most common manifestations of the parasitic twin take the form of an individual twin with extra limbs or organs. This can lead to debates about whether or not the dominant twin should be classified as a parasitic twin at all or if they would be better described a non-twin person with physical mutations.
These cases can also overlap with Vanishing Twin Syndrome which occurs when one twin’s fetal tissue is absorbed by the other.
Another notable type of parasitic twin is called “fetus in fetu,” or literally “fetus inside fetus.” As you might have guessed, fetus in fetu describes an underdeveloped fetus located inside of the dominant twin.
For the dominant twin, they often remain asymptomatic until they are older. These cases of incomplete twinning are very uncommon and continue to remain medical curiosities today.
Superfetation occurs when a person becomes pregnant a second time while already pregnant. Less than 20 cases have ever been recorded involving humans, but it is actually quite common in animal species such as fish or badgers.
During superfetation, the first pregnancy works like a normal birth, but the second pregnancy has a slightly different timeline. The second pregnancy requires a new fertilized egg to be inserted into the womb days or weeks later than its twin.
It is important to note that unlike the other twin variations discussed above, superfetation twins are fraternal twins — not identical twins — since they develop from two separate eggs instead of a single shared egg that separates later on.
Additionally, superfetation causes the twins to grow at different rates. While the twins can still be born on the same day, the twin that was conceived later will be born prematurely, at a smaller size or weight. Complications can arise, but medical professionals can take steps to monitor and care for the premature twin accordingly.
Twins don’t just have to be labelled as identical or fraternal. There are other types of unusual twins that exist as well.
While these circumstances can sometimes pose risks to the mother or the twins themselves, these unique twins can still grow up to live fulfilling, healthy lives.
Cafasso, Jacquelyn. “Superfetation.” Healthline, Healthline, 26 Mar. 2018, www.healthline.com/health/superfetation.
DeRuiter, Corinne. “Parasitic Twins.” The Embryo Project Encyclopedia, The Embryo Project Encyclopedia, 16 Aug. 2016, embryo.asu.edu/pages/parasitic-twins.
Fletcher, Dan. “How Can a Pregnant Woman Get Pregnant Again?” Time, Time Inc., 28 Sept. 2009, content.time.com.
Gill, Karen. “Types of Twins.” Healthline, Healthline, 22 Apr. 2019, www.healthline.com/health/pregnancy/types-of-twins.
Smith, Jodi. “Here’s What Happens In The Tragic Case That One Conjoined Twin Dies.” Ranker, Conjoined for Life, 14 June 2019, www.ranker.com/list/conjoined-twin-death-facts/jodi-smith.
Zapata, Kimberly. “What Are Mirror Twins? Here’s Everything You Want to Know.” Healthline, Healthline, 21 Aug. 2020, www.healthline.com/health/pregnancy/mirror-twins.