When it comes to twins, you probably think of either Dizygotic (fraternal) twins that result when two eggs are fertilized or Monozygotic (identical) twins that come from a single fertilized egg that splits. But did you know there are other rare and unique types of twins? Check out our run-down of these unique twin types that are pretty unusual and rare in our twin world.
Polar Body Twins or “Half Identical”
What happens when the egg splits and then each half meets a sperm? That’s what scientists and researchers propose happens when a polar body or “half-identical” twins form. These twins are very much alike but aren’t a 100% DNA match — sharing about 75% of their genetic DNA. (Less than identical twins but more than fraternal twins.) To date, polar body twinning remains a theory. Believe it or not, there are no definitive tests to confirm whether you are carrying polar body twins even though there have been confirmed cases.
This unique twin type identifies as identical on the mother’s side but shares only half their father’s genes! Semi-identical twins develop when two sperm fertilize a single egg, forming a triploid, which then splits. In 2007, the Journal of Human Genetics described a case where one hermaphrodite twin (someone born with a discrepancy between their internal genitalia and external genitalia) was raised as a female, with both testicular and ovarian structures, while the other was anatomically male.
Boy/Girl Monozygotic (Identical) Twins
Did you know identical twins are almost always the same gender because they form from a single zygote containing male (XY) or female (XX) sex chromosomes? However, there are a few reported cases of a genetic mutation in some male twins where one twin loses a Y chromosome and develops as a female. The female twin would then be afflicted with Turner’s Syndrome, a genetic disorder in girls caused by a missing or defective X (female) chromosome. It occurs in 1 of 2,000-2,500 live female births. There are many different features associated with the syndrome, and not all girls have all symptoms. Still, typically they are short in stature and lack ovarian development, have hearing disorders, abnormal fingernails, toenails, and drooping eyelids (Ptosis). The only other time gender differences would occur in identical twins would be if one twin experienced a gender transition process.
Mirror Image Twins
Mirror image twins make up around 25% of identical twins. This unique twin type occurs when the fertilized egg splits late – more than a week after conception. This type of twins can develop reverse asymmetric physical features, such as birthmarks or swirling hair whorls on opposite sides of their bodies. So, when these types of twins face each other, they appear to be exact reflections (like looking in a mirror).
Superfetation: Twins Conceived Separately
Typically, when a woman’s egg is fertilized, her cycle is interrupted, and her ovulation ceases. Rarely, an egg can be released while she is already pregnant, resulting in twins that are conceived at different times! This happens when eggs from two separate menstrual cycles are released as opposed to normal fraternal (Dizygotic) twins, where multiple ova are expelled in a single cycle. This is a rare occurrence in humans but is common in animals and usually results in twins or higher-order multiple pregnancies where the fetuses show noticeable differences in gestational development. Sounds crazy, but this Arkansas mom was pregnant with two babies conceived about two and a half weeks apart.
Heteropaternal Superfecundation: Twins with Different Fathers
Yes, you read that right. Twins with different fathers are possible. Sure, the definition for twins clearly states, “Twins are two babies who are simultaneously born from one mother.” But what about the dad? We know that when a woman releases multiple eggs in a single cycle, known as hyper-ovulation, it results in fraternal twins. However, superfecundation happens when the eggs are fertilized by sperm from separate incidences of sexual intercourse. In a nutshell: A woman with multiple sexual partners could have twins with different fathers (heteropaternal superfecundation). Genetic testing and advances in technology have made it evident that twins with two fathers only apply to fraternal twins, not identical twins. This situation can also occur when twins are the result of fertility treatments. This happened in 1995 with a pair of IVF Dutch twins. There was a mix-up in the lab when a piece of lab equipment had been accidentally used twice, causing another man’s sperm to be mixed in with the father’s and fertilized with the mother’s sperm. The result: two babies of different races.
Twins of Different Races: So rare that the odds against this are a million to one!
As mentioned above, heteropaternal superfecundation explains cases of fraternal twins with differing racial characteristics. The case above was due to a lab mix-up during an IVF procedure. However, in the United Kingdom back in 2005 and described as a “one in a million” occurrence, fraternal twin girls were conceived from two bi-racial parents. Each inherited different genetic characteristics from their mixed-race parents. (Essentially, one was white, and the other was black.) For this rare occurrence to take place, there must be a combination of circumstances starting at the exact moment of conception. This phenomenon can occur when both parents are of mixed race; each is conceived from a separate egg fertilized by a separate sperm (fraternal); and each sperm and egg must carry the genes for particular skin color (i.e., black/black or white/white).
In short, conjoined twins are identical twins that do not fully separate from each other due to the incomplete division of the fertilized egg. The individuals will be connected to specific body parts and may share tissue, organs, or limbs. As the Children’s Hospital of Philadelphia states, “Conjoined twins occur once in every 50,000 to 60,000 births.”
Parasitic twins are conjoined twins that develop asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. In Cairo, Egypt, back in 2005, a case of parasitic twins gained worldwide notoriety and even featured on The Oprah Show. Manar Maged was born with a second head attached to the skull of her own. While the head could blink and smile, it was not capable of independent life. The weight of the appendage would prevent Manar from crawling or sitting upright, prompting surgeons to remove it when she was ten months old. On February 19, 2005, doctors performed a thirteen-hour surgery to remove the attached head. The girl was released from intensive care in March 2005. Unfortunately, she passed away from a brain infection shortly before her second birthday on March 25, 2006. A variation of parasitic twinning is called “the fetus in fetu.” This occurs when an abnormally formed mass of cells grows inside the body of its identical twin. These cells survive during pregnancy and even occasionally after birth by tapping directly into the host twin’s blood supply.
Curious to find out more? Check out this report on a man who discovered a fetus in fetu as an adult.
The bottom line
Although most of us are familiar with fraternal and identical twins, many aren’t aware of the other unique twin types out there. After reading about these rare and unique twin types, you may have discovered a whole range of unique twin combinations that you may not have known were possible. But one fact remains true, regardless of what type of twin you are or have, the birth of twins is a miracle indeed.
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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.
To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information. To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.What is unique about being a twin? ›
Twins don't share the same fingerprints for example because even though they share a womb, they sit in different places in the womb, which gives them a different set of prints. They also don't share the same DNA because studies have found that identical twins can have different DNA sequences.Are boy girl twins rare? ›
Mixed-gender twins are the most common type of fraternals, some 50 percent are boy-girl.How rare is it to be a twin and 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.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 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.
Octuplets are a set of eight offspring born at one birth. An individual that is part of such a set is called an octuplet.What kind of twins are the most popular? ›
Fraternal twins are the most common type of twins, accounting for around two thirds of all twin pregnancies. Fraternal twins are the most common type of twins, formed when two eggs are fertilised by two separate sperm. They have as much in common as siblings who were born separately.What is the most common type of twin? ›
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.
Two babies are twins and three are triplets.Can a twin marry a twin? ›
This rare phenomenon is known as quaternary twins. It happens when identical twin females marry identical twin males and have babies at the same time. Twins Briana and Brittnay Deane, 35, who were inseparable since birth got hitched with twins Josh and Jeremy Salyers.What's good about having a twin? ›
What are the benefits for the twins? Their buddy is always there. They've known each other since they can remember and that is very comforting. They are also a little more social than the average child since they are used to being with another kid since the beginning.How does being a twin impact your life? ›
Being a twin has its benefits—tricking people, having a lifelong companion, sharing clothes—but it also has downsides. Many twins struggle to cultivate their own identities, while being so similar to one another. And that struggle lasts a lifetime, according to a recent study.Are twins born the same gender? ›
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.Can twins have different fathers? ›
But you may be left wondering: Can twins have different fathers? The answer is yes, but only in cases in which they're fraternal, as identical twins form from a single egg/sperm combination and thus cannot have different fathers.What gender are twins usually? ›
Identical twins are almost always the same sex, although there are some rare exceptions to this rule. Because identical twins occur when a single fertilized egg splits and forms two embryos, each embryo has the same chromosomes (usually, XX for girls or XY for boys). Remember that sex and gender are not the same.Why twins are so rare? ›
Variation in gonadotropin levels may explain this trend and may also represent the genetically determined mechanism for the heredity of twinning (Vogel and Motulsky 1986).Can 2 twins have a baby? ›
Science has shown if 2 sets of identical twins have kids, the kids will be genetic brother/sisters of each other. If you somehow had 2 identical twins (not fraternal) that could produce a baby. The offspring would be a clone of the parents. It would be a genetically identical clone.Can both twins be born at the same time? ›
Twins are defined as two offspring born together, but that doesn't necessarily mean they are born on the same date. Multiples are generally born only a few minutes apart.
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.What are half twins called? ›
The fertilized egg then splits in two. So, semi-identical twins share all the same chromosomes from their birthing parent, but only about 50 percent from their non-birthing parent.
Some names for these are: Twins for 2 fetuses. Triplets for 3 fetuses. Quadruplets for 4 fetuses.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'.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.How many kids can a woman have? ›
One study estimated a woman can have around 15-30 children in a lifetime, taking pregnancy and recovery time into account. Since men require less time and fewer resources to have kids, the most "prolific" fathers today can have up to about 200 children.What are mirror twins called? ›
The term mirror twin is used to describe a type of identical, or monozygotic, twin pairing in which the twins are matched as if they're looking into a mirror — with defining characteristics like birthmarks, dominant hands, or other features on opposite sides.Why are twins so common now? ›
About 1.6 million twins are born each year and one in every 42 babies is a twin. A big reason for all those twins is an increase in medically assisted reproduction, including in vitro fertilization, ovarian stimulation and artificial insemination.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.
Identical twins turn out as the same gender about 99.9% of the time.Is there a twin for everyone? ›
Believe it or not, scientists say that statistically, every person has roughly SIX doppelgangers out there in the world. That means there are seven people with your face, including you, out there.Do twins come from mom or dad? ›
For a given pregnancy, the odds of conceiving fraternal twins are only determined by the mother's genetics, not the father's. Fraternal twins happen when two eggs are simultaneously fertilised instead of just one.What increases chances of twins? ›
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.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.Can you be pregnant while pregnant? ›
Called a superfetation, getting pregnant while already carrying a baby is so rare that one 2008 study found fewer than 10 recorded cases in the world. Doctors told the couple the babies were actually conceived about three weeks apart, Rebecca said.What is six twins called? ›
Thanks to reality TV, we've heard of quintuplets (five babies at once), sextuplets (six babies at once), and other higher-order multiples. In 2021, a Malian parent gave birth to nine babies (nonuplets).Which is more rare identical or fraternal twins? ›
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.What is a boy and girl twin called? ›
If twins are a boy and a girl, clearly they are fraternal twins, as they do not have the same DNA.
Can twins have different fathers? In rare cases, fraternal twins can be born from two different fathers in a phenomenon called heteropaternal superfecundation. Although uncommon, rare cases have been documented where a woman is pregnant by two different men at the same time.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 is it called when your twins but don't look alike? ›
These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins. During pregnancy, the developing babies get oxygen and food from their mother through the placentas and umbilical cords.Can identical twins be boy and girl? ›
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.Can 2 sperm fertilize the same egg? ›
Occasionally, two sperm are known to fertilize a single egg; this 'double fertilization' is thought to happen in about 1% of human conceptions. An embryo created this way doesn't usually survive, but a few cases are known to have made it — these children are chimaeras of cells with X and Y chromosomes.Do twins have the same blood type? ›
Identical twins will always have the same blood type because they were created from the same fertilized egg (fraternal twins can have different blood types — again, providing the parents do — because they are created by two fertilized eggs).