When the question, “Are they twins?” is answered with a yes, you can pretty much guarantee that the next question will be, “Fraternal or identical?” We like to place people into categories, and there is no easier way to do that than by keeping it to as few options as possible. But the real world is a much more complex place, and when it comes to twins, there are many more options than simply A or B.
The purpose of a woman’s reproductive cycle is quite simple: to get pregnant. So, when that goal has been achieved, the body stops dropping eggs and shifts its attention to nurturing the fetus. This usually happens within ten days of getting pregnant, meaning that unless multiple eggs were dropped, you are probably as pregnant as you are going to get. Probably.
Superfetation is the phenomenon of a woman getting pregnant when she is already pregnant. With only ten recorded cases of superfetation in human history, to describe it as rare would be an understatement. What makes superfetation so rare is that it requires the body to “malfunction” in two very specific ways. Firstly, the uterus has to remain open for longer than the usual ten days in order to allow a second egg in. On top of that, the second egg must drop much sooner than expected. It is usually one egg every 28 days, but this can vary. Even if both of these things do happen, the woman still has to go through all the usual motions of getting pregnant.
The biggest risk with superfetation is that the second baby will be born prematurely when the mother goes into labor with the first child, but since the children are conceived within weeks of each other, this is not a major issue. In fact, the likelihood is that this has happened more than we realize, but that the parents simply assume the twins are not identical.
Heteropaternal Superfecundation is a rare type of twins, not so much because of the genetic factors involved, but because of the mating habits of humans. This occurs when a woman has dropped two eggs, which are then fertilized by different men. Needless to say, it is usually, if not always, unintentional, such as in this case we previously examined, but it would be much more common if we were less monogamous as a species.
Heteropaternal Superfecundation is actually quite common in other species, such as cats, where the mother simply wants to have as many babies as she can. Taking as many partners as possible will maximize the chances that any eggs dropped will be fertilized, and if dad is not hanging around to help anyway, it doesn’t really matter who the father is. This explains why a litter of cats can vary so much in patterns and color, and also contributes to the fact that inbreeding between animals is less of a risk, as there is a greater genetic variance between relatives than in humans.
The sex of identical twins is as random as it would be with any normal pregnancy. During conception an X or Y chromosome from the sperm fuses with the X chromosome from the egg, producing either XY (male) or XX (female). So when an egg is fertilized, the sex is determined immediately, and the egg later splits up into two children of the same sex.
However, an unrelated medical condition can cause identical twins to be born different sexes. Mosaic Turner Syndrome is when a developing fetus loses its second chromosome early on in development due to genetic abnormalities. Such people are classified as X0, and since there is no Y chromosome there to make them male, they end up female. When born, the twins will simply look like opposite-sex fraternal twins, although women with Turner Syndrome will have developmental and reproductive issues as she grows, meaning the phenomenon is usually identified eventually. Touchingly, the fact that there is another fetus in the womb gives the one with MTS a better chance of survival.
Identical twins form when a sperm penetrates an egg, and in the early stages of development, the egg splits in two. Since it was the same sperm and the same egg, the result is that their genetic makeup is identical. Although different sperm can carry different genetic information, once an egg has been fertilized, it cannot be fertilized a second time. But what if two sperm managed to break their way into the egg at literally the exact same moment?
This is known as semi-identical twinning, and it was first documented in 2007. This case first came to the doctor’s attention when it was discovered that one-half of a pair of twins was a hermaphrodite. Upon closer inspection, it transpired that two sperm from the same man had penetrated the one egg simultaneously, in the most literal sense of the word. This usually results in an unsuccessful pregnancy, although there have been cases where these fetuses have been carried to term. In the 2007 case, however, things went a step further, and the egg split in two. This resulted in each twin getting a little bit of genetics from each sperm, making them half-fraternal and half-identical.
6Polar Body Twins
Also known as half-identical twins, polar body twins are a theoretical form of twins that have never actually been documented by science. However, there really is nothing preventing this from occurring; it would just require constant observation during conception to confirm.
The theory states that polar body twins form when an egg drops and then splits in two. The smaller of the two new eggs is referred to as the polar body, which rarely survives due to a lack of cytoplasm, which is every part of a cell excluding the nucleus. There is no reason a polar body could not be fertilized and carried to term; it would just be more of a struggle as there is less material to work with. If both the primary and polar body were fertilized, the result would be half-identical twins, which would share 75 percent of their genetics, as opposed to the usual 50 percent seen in fraternal twins.
Mirror-Image twins are without a doubt the most common type of rare twins, with an estimated 1 in 4 pairs of identical twins being classed as such. They are also considerably easier to spot than many of the other entries on this list, so much so that it would be near impossible not to notice if you met a pair.
As you may have guessed, mirror-image twins are those who are opposites of each other. One is left-handed, the other, right; their hair will swirl in the opposite direction; even genetic physical defects will appear on opposite sides. This occurs when the egg splits between the 7 and 12 day mark. Any later, and the twins would likely be conjoined. Any earlier, and they would just be boring old identical twins.
4Mixed Chromosome Twins
Mixed chromosome twins are rare, but the truth is, we don’t really know how rare. This is because, unlike most twins, there is only one baby at the end of the journey. So how can they be a twin? Well, we know that identical twins occur when a fertilized egg splits in two, but what if this process were reversed, and two fertilized eggs merged? Then we would get mixed chromosome twins, otherwise known as chimeras.
Chimeras can form in a few ways, such as a viable embryo absorbing one that fails. In the case of mixed chromosome twins, they start out as a set of fraternal twins, but early on in the pregnancy, the two fertilized eggs merge into one, resulting in a person with two distinct sets of DNA. This is often unnoticeable without extensive DNA testing. Other times, chimeras will have very distinct features, such as chequered skin with different tones. The different sets of DNA, which can even be different sexes, will be located in various parts of the body, so a test from your blood could turn up different results to a swab from your mouth. Two women in the US almost lost custody of their children after failing DNA tests, with the most recent woman, Lydia Fairchild, being dubbed “her own twin.”
Parasitic twins are a form of conjoined twins, which occur when a fertilized egg fails to split properly. Unlike conjoined twins, where two distinct people with separate sets of consciousness are joined together, a parasitic twin is smaller, not fully formed, and is not a conscious being.
Parasitic twins can develop anywhere on the body and can come in a variety of forms, from a small, indistinct lump, to a second (defunct) head, and even extra limbs growing out of different parts of the body. In some cases, removing the parasite can be quite straightforward, while in others, attempting to do so could be life-threatening or simply impossible.
Biracial twins, meaning twins that are each a different race as opposed to both being mixed-race, can happen in three ways. The first is when two parents of different ethnicities have fraternal twins, which we have discussed before. Quite simply, one twin takes after the father, while the other takes after the mother. The second is Heteropaternal Superfecundation, where at least two of the parents are different races. For example, if a white woman was with an Asian man and a white man, one baby will definitely be white, while the other could appear white, Asian, or mixed-race.
The third way it can occur is if two mixed-race people have fraternal twins, which is how a pair of twin girls in the UK were born different ethnicities. In that case, two half-white, half-black parents had a set of twins, with one picking up all the white features, and the other picking up the black features. This is another example of something that would not be too uncommon if our mating habits were different. With more and more interracial relationships occurring these days, instances of biracial fraternal twins will inevitably become increasingly more common as time goes on, as each child’s genetic makeup is essentially a roll of the die completely unrelated to the egg floating next to it.
We have established that twins can have different fathers, but it turns out they can technically have different mothers too. There has only been one instance of this, in 2007, and it really was the mother of all coincidences. After struggling for 12 years to conceive a second child, the Bernabas decided to undergo both IVF and surrogacy. Against all odds, the fertilized eggs took hold in both women, meaning that Mrs. Bernaba and her surrogate, Ms. Keay, were each carrying one-half of a pair of fraternal twins.
Even more shocking is the fact that when Mrs. Bernaba delivered her child by C-section on May 27, her husband was told not to get too comfortable with baby Lauren just yet, as Ms. Keay had gone into labor in the next room. Soon after, still on the same day, baby Hannah was delivered. While this remains the only case of its kind so far, the increasing popularity and variety of fertility treatments, such as three-parent children, means nobody can say for sure what the future will consider normal.
Simon has no twin, but he has a Twitter @simongireland
Read more about twins on 10 Extraordinary Tales of Extraordinary Twins and 10 Terrible Tales Of Murderous Identical Twins.
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fact checked by Jamie Frater
What is the rarest type of twins? ›
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
There are more than just fraternal and identical twosomes; other rare twin types include “half identical twins” (when the egg splits and then each half is fertilized) and mirror image twins (identical twins who develop asymmetrically). 3. Identical twins don't have identical fingerprints.What is the most popular type of twins? ›
Dizygotic twins are the most common type and are known as fraternal twins. Two-thirds of all sets of twins are dizygotic. When two eggs are fertilized during the same pregnancy, the result is a set of dizygotic twins. They are fertilized by two different sperm.What is a super twin baby? ›
Superfetation is when a second, new pregnancy occurs during an initial pregnancy. Another ovum (egg) is fertilized by sperm and implanted in the womb days or weeks later than the first one. Babies born from superfetation are often considered twins since they may be born during the same birth on the same day.How rare are 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?What are 10 twins called? ›
– Decuplets: a combination of 10 of a kind. In this case, babies! A South African woman has reportedly given birth to 10 babies at once, besting a world record set just last month.What are 12 twins called? ›
If the 12 – called duodecaplets – are all born alive they would represent a medical miracle and break the record of American mother Nadya Suleman, who recently gave birth to the world's longest-surviving octuplets.What are 9 twins called? ›
A set of nonuplets was born on 13 June 1971, in Sydney, Australia to Geraldine Brodrick and her husband Leonard. They were five boys and four girls; two of the boys were stillborn and the last of the surviving babies, a boy named Richard, died six days after the birth.
Sometimes twins develop their own individual sacs, but when they share one, it increases the likelihood that one twin could be hiding during an early ultrasound. These twins are called monochorionic monoamniotic (or mo/mo twins, for short). Your hidden baby is super stealthy.What is the magical twins name? ›
When the King is taken prisoner by Tartarath, the “Dark Master,” twins Prince Aram and Princess Mara, set out to rescue their father and save the kingdom. But when the use of magic is forbidden, the twins will have to find new ways to prove themselves worthy of restoring the throne. For ages 13+.
What are famous twins? ›
Consuela and Allie Jones are known as the Famous Twins through their handle @fam0us. twinsss on Instagram and TikTok, where they have a million followers.Are twins rarer than triplets? ›
Naturally, twins occur in about one in 250 pregnancies, triplets in about one in 10,000 pregnancies, and quadruplets in about one in 700,000 pregnancies. The main factor that increases your chances of having a multiple pregnancy is the use of infertility treatment, but there are other factors.Which twin is older? ›
Which twin is older? The oldest twin is the one who is born first. It doesn't matter whether twins are born vaginally or by C-section; the first one out of the womb is considered the oldest. This may be the baby who was labeled baby A during the ultrasounds, but it might not if the other one makes it out first.What rare birth only happens every 480 years? ›
The Czech Republic birth data revealed that quintuplets are born once every 480 years! Hence this is an exceptionally great birth and all the kids and the mum are healthy!What is the first born twin called? ›
Throughout pregnancy, we refer to twins as 'first and second' or 'left and right' or 'the cephalic one', 'the breech one', 'the transverse one'.Can you get pregnant while pregnant? ›
A double pregnancy, or superfetation, is extremely rare — in fact, there aren't even stats on how often it happens — but it's scientifically possible. We're not saying you should worry about it happening to you, just that you can't say that it's impossible.Is Vanishing twin rare? ›
Vanishing twin syndrome is evaluated to happen in 36% of twin pregnancies and in half of the pregnancies that begins with at least three or more gestational sacs.What is a chimera twin? ›
Chimerism occurs when a woman is pregnant with twins and one embryo dies, and the other embryo absorbs the twin's cells. (Scientifically speaking, this type of chimerism is called tetragametic because the baby was derived from four gametes – one egg and one sperm for each embryo.)What are polar twins? ›
Polar body twinning is thought to occur when an egg splits - and each half is fertilised by a different sperm. This results in twins who appear very much alike but share approximately 75% of their DNA.Who gave birth to 9 babies at once? ›
Remember the African woman who set a Guinness World Record last year when she gave birth to nine babies at once? A woman named Halima Cissé, from Mali, gave birth to nine babies, five girls and four boys, in Morrocco's port city of Casablanca.
What do you call triple twins? ›
If you're carrying two babies, they are called twins. Three babies that are carried during one pregnancy are called triplets. You can also carry more than three babies at one time (high-order multiples).What twin names go together? ›
- Olivia and Sophia.
- Gabriella and Isabella.
- Heaven and Nevaeh.
- Arianna and Brianna.
- Elizabeth and Isabella.
- Paige and Natalie.
- London and Paris.
- Elizabeth and Maria.
There have only been two reported cases of semi-identical twins, so this type is extremely rare. In semi-identical twins, two separate sperm fertilize one egg. The fertilized egg then splits in two.Are twins 100% the same? ›
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.Can twins be half siblings? ›
Homopaternal superfecundation is fertilization of two separate ova from the same father, leading to fraternal twins, while heteropaternal superfecundation is a form of atypical twinning where, genetically, the twins are half siblings – sharing the same mother, but with different fathers.Which twin lives longest? ›
Identical twins live longer, possibly because they have a BFF from birth, according to researchers from the University of Washington, Seattle, WA. Furthermore, this effect is greatest in male identical twins, according to results from their recent study, published in the journal PLOS ONE.What's the biggest twin born? ›
The worlds heaviest twins, with an aggregate weight of 12.58 kg (27 lb 12 oz), were born to Mary Ann Haskin (USA), of Fort Smith, Arkansas, USA on 20 February 1924.Has anyone ever had 10 babies at once? ›
But have you heard of a woman giving birth to 10 babies at once? There were reports that this South African woman gave birth to 10 babies and broke Guinness World Record, however, this is not true. There were stories claiming that the woman broke the record. A fact check by Lead Story concludes that the claim is wrong.Is being a twin rare? ›
It is estimated that 1 in 250 natural pregnancies will naturally result in twins. While twin pregnancies can happen by chance, there are some factors that may increase your odds of having two babies at the same time. Let's learn about twins!What are 4 twins named? ›
Some names for these are: Twins for 2 fetuses. Triplets for 3 fetuses. Quadruplets for 4 fetuses.
What are Mexican names for twins? ›
Double names are very important in Hispanic cultures. Mexican most common double names start with Juan, José, Luís and Miguel, the favorite combinations being Miguel Ángel, Juan Pablo, Juan Manuel, José Luís and José Miguel.What is a ghost twin? ›
It occurs when an embryo detected during an ultrasound can't be found on a future ultrasound. The embryo stops developing, and its tissue gets absorbed by the mother, or gestational parent, and the surviving embryo(s).What is a rainbow twin? ›
A rainbow baby is a child born after the loss of a previous pregnancy or newborn. Read Caitlin's story, and learn about her two miraculous rainbow babies. You've heard of a rainbow baby, a child born after the loss of a previous pregnancy or newborn.What happens when a twin vanishes? ›
One baby was miscarried during the pregnancy without the mothers or doctors knowing. Doctors called these cases vanishing twins or vanishing twin syndrome (VTS). The tissue from a vanishing twin is mostly reabsorbed by the mother's body and the remaining baby. Sometimes some evidence remains.Who are the demon twins? ›
The Demonic Twins (or Worldly Chivalrous Duo as they call themselves) of the Seven Demons are a pair of old twins who Noh Gung Hyun and Noh Gung Myung met in the Heavenly Dragon Castle.Who are the immortal twins? ›
In Latin the twins are also known as the Gemini (literally "twins") or Castores, as well as the Tyndaridae or Tyndarids. Pollux asked Zeus to let him share his own immortality with his twin to keep them together, and they were transformed into the constellation Gemini.Which god pairs are twins? ›
- Aegyptus and Danaus (Greek)
- Aeolus and Boeotus (Greek)
- Agenor and Belus (Greek)
- Amphion and Zethus (Greek)
- Apollo and Artemis/Diana (Greek) (Roman)
- Arsu and Azizos (Palmyran)
- Ascalaphus and Ialmenus (Greek)
- Atreus and Thyestes (Greek)
Two-thirds of all twins in the world are fraternal. Uniform around the world; about 3 in 1,000. Only one-third of all twins in the world are identical. Develop separate sacs in utero.What type of twins have 100% of their genes in common? ›
Identical, or monozygotic (MZ), twins have 100 percent of their genes—including those that influence risk for alcoholism—in common, whereas fraternal, or dizygotic (DZ), twins share (on average) only 50 percent of the genes that vary in the population (see figure). Common Environmental Sources.Which type of twins skip a generation? ›
Many people believe twins skip a generation, but that's just a myth. The idea that twins skip generations likely comes from the fact that the genetic factors contributing to twins only come from the gestational parent's side.
What are the rarest multiple births? ›
Identical triplets or quadruplets are very rare and result when the original fertilized egg splits and then one of the resultant cells splits again (for triplets) or, even more rarely, a further split occurs (for quadruplets).What is Chimera twin? ›
When two zygotes do not undergo fusion but exchange cells and genetic material during development, two individuals, or twin chimeras, one or both of whom contain two genetically distinct cell populations, are produced. The most widely known examples of twin chimerism are blood chimeras.What are Irish twins? ›
Irish twin (plural Irish twins) (rare, slang, offensive) Either of a pair of siblings born less than 12 months apart, especially if born within the same calendar year or school year or born one year apart.Can twins share 75% DNA? ›
Identical twins have 100% of their DNA the same while fraternal twins share only 50%. But what about semi-identical twins? Well, they have 75% of their DNA the same.Can twins have twins? ›
According to The American Society for Reproductive Medicine, women who are fraternal twins have a 1 in 60 chance of having twins, and men who are fraternal twins have a 1 in 125 chance of fathering twins. It was previously believed that identical (monozygotic) twins were random — not genetic.Can a boy and girl be identical twins? ›
In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.What are reverse twins? ›
Mirror twins, or mirror image twins, is a term used to describe a characteristic of some twins whose features appear asymmetrically—that is, on opposite sides. When these twins are facing each other, it is as if they are looking in a mirror.Which parent decides twins? ›
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.Which twin runs in the family? ›
Dizygotic (DZ) twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle. DZ twins are about twice as common as MZ twins, and they are much more likely to run in families.What is 9 twins called? ›
1.5 Sextuplets (6) 1.6 Septuplets (7) 1.7 Octuplets (8) 1.8 Nonuplets (9)
What is 8 twins called? ›
Octuplets are a set of eight offspring born at one birth. An individual that is part of such a set is called an octuplet.