What Are The Six Mysterious Twin Types In Humans


Table of Contents

How Many Twin Types Exist In Humans

Six twin types are currently known to exist in humans. Twins are complicated and their creation a genuine miracle and mystery that humanity is trying to decode and understand. You might be familiar with the two most common types of twins, identical twins and the non-identical fraternal kind. However, other rare subtypes exist, and some are so rare they have a name, but science cannot scientifically explain how they occur.  

Before reading this article, I strongly recommend you watch the SciShow video. It will give you a clear explanation everyone will understand and graphics that make digesting the written material a great deal easier.

SicShow Video link: https://www.youtube.com/watch?v=bpWJNSBQo0k

What Are The Normal Twin Types

Monozygotic or identical (MZ)

Dizygotic, fraternal, or non-identical (DZ)

MZ twins develop when a single sperm fertilise one egg, and during the first two weeks after conception, the developing embryo splits into two. As a result, two genetically identical babies develop.

DZ twins occur when two eggs are released in a single ovulation and fertilised by two different sperm. These two fertilised eggs then implant independently in the uterus. DZ twins share the same genetic relationship as non-twin siblings, hence the term fraternal.


What Are The Six Types Of Mysterious Twins Sets

  • Mirror image twins
  • Conjoined twins
  • External heteropagus twins
  • Fetus-in-Fetu (200 reported cases)
  • Chimeric twins
  • Sesquizygotic twins (Only 2 reported cases in the world to date)

 Identical Twins

Monozygotic identical twins are conceived from one fertilised egg. This egg separates into two embryos after it has begun to divide. These two embryos develop into two babies.

Genetic materials called chromosomes in both babies are completely identical. This is because both babies come from the same egg and sperm. For this reason, both children are assigned the same sex at birth and share the same genetic characteristics, such as eye and hair colour.

Still, identical twins may have slight differences in appearance because of differences in the environment where they’re born (like the amount of space each had in the uterus).

If the existing twins are identical, the chance of having another set is the same as in most women, about 1 in 250. If the twins are non-identical, the chances of having twins again are much higher.

Non-identical Twins

Not all twins are identical. Even identical twins can have minor differences. Twins are born with unique physical characteristics. Non-identical twins are generally known as fraternal twins.

Fraternal Twins

The scientific term for fraternal twins is “dizygotic” and refers to two fertilised eggs via two separate and different sperm. Dizygotic twins happen when the ovary/s release two eggs at the same time. A different sperm will fertilise each egg, making it possible to conceive two genetically unrelated paternal twins by engaging in coitus with two men who release sperm into the uterus within hours of each other or at the same time.

While fraternal twins can be from two different fathers, they more commonly share the same paternal and maternal genetic DNA material.

Fraternal twins result from different eggs and sperm, meaning they share the same percentage of chromosomes as any other siblings. Current research suggests fifty per cent, which is why siblings don’t look exactly alike but can all look similar and easily be recognised as being related. It also explains why siblings can be assigned different sexes at birth, as the males determine the sex of a child through their XY chromosomes while women have XX. One chromosome is taken from both parents.

The mother can only provide an X chromosome. If the father provides an X, they will have a female. If the father provides a Y chromosome, they will have a male.

In rare cases, a third option is available. Klinefelter syndrome is a genetic condition resulting from a boy’s birth with an extra copy of the X chromosome. Klinefelter syndrome is a genetic condition affecting males and often isn’t diagnosed until adulthood.


Is There A Third Type Of Twin

Traditionally, the science around twins has taught that identical and fraternal are the only two types. But a third type might exist, called a polar body or half-identical twins.

Though this has never been confirmed, research from 2016 that is still ongoing suggests that a third twin type would explain why some fraternal twins look so similar.

After the ovaries release an egg, the egg splits into two halves. The smaller half is called a polar body. This polar body contains all the chromosomes necessary to join with a sperm to create a baby. But since it usually contains very little fluid or cytoplasm, it is theorised to be too small to survive.

While improbable, it is not impossible that a polar body could survive and be fertilised. Meanwhile, the larger half of the original egg could also be fertilised by a separate sperm resulting in Polar twins.

In theory, Polar twins share the same chromosomes from their mother, but they get different chromosomes from the father. This is because they are created from a single egg but fertilised by two separate sperm.

For this reason, they may or may not be assigned the same sex at birth and may look very similar but not identical.


 Unique Identical Twins

In a basic twin pregnancy, two embryos go their separate ways and develop into twin babies, whether identical or fraternal. Some unique sets of twins follow a different path.

Mirror Twins

Mirror twins are exactly what they sound like! These twins are actual mirror images of each other. This means that:

  • Their hair may naturally fall in opposite directions.
  • One twin will have their vital organs on the opposite side of their body to the other twin. This is called Situs Inversus. https://my.clevelandclinic.org/health/diseases/23486-situs-inversusThey may have identical birthmarks on the opposite side of their bodies.
  • They also usually have different dominant hands—one left-handed and one right-handed.
  • They are for all intent and purpose, like looking into a mirror inside and outside.

In a typical identical twin pregnancy, an egg splits during its first week after fertilisation. But in a mirror twin pregnancy, the egg splits 7 to 12 days after fertilisation, meaning that it has developed a right and a left side.

Conjoined Twins

Conjoined twins are a rare twin type in which the two siblings are physically connected. Typically, conjoined twins are joined together at the head, chest, or abdomen, but this can vary in severity and the number of surfaces conjoined. Most conjoined twins share at least one vital organ, and some will share several, depending on what stage the fusion took place.

Though physically fused to each other at some point, conjoined twins are two separate individuals. They have unique thoughts and their own personalities like the rest of us.

Researchers are still left scratching their heads, trying to understand the origins of this type of twin birth. Some experts believe conjoined twins occur when a fertilised egg doesn’t split completely. This happens when the egg divides 12 or more days after conception. 

Another theory suggests that the fertilised egg divides completely but later fuses back together. If you have not already watched the SciShow video linked at the top of the article, I suggest this is the perfect time to view it, as this part will make far more sense.

A conjoined twin pregnancy is high-risk and requires a Caesarean birth. Post-birth surgery allows the majority of conjoined twins to be separated and live independent lives.

Studies suggest only 7.5 per cent of conjoined twins live past birth in developed countries, and next to none make it out of the womb alive in third world countries or only survive a few hours without the intense neonatal care and equipment required. Often, sadly, conjoined twins who are not surgically removed via C-section will kill the mother and themselves in the long and excruciating failed birthing process.

Parasitic Twins

Sometimes, as twins develop in utero, one twin becomes larger and more dominant. The other twin stops developing and begins to depend on their sibling. Known as parasitic twins, these twins are physically conjoined. The smaller twin is not fully formed and isn’t capable of surviving on their own. This is believed to be due to the smaller twin missing vital organs or a fully developed brain or heart.

Shockingly, you might not recognise the smaller twin as a separate individual. This “twin” may appear on the sibling’s body as a small lump, extra limbs, or a second non-functioning head.


Parasitic twins subtypes include fetus-in-fetu and acardiac twins:

Fetus-in-fetu. These are rare circumstances when a smaller twin develops inside the larger twin’s body.

Acardiac twins. In this case, one twin receives too much blood flow while the other doesn’t receive enough. This occurs because identical twins share a placenta.

A milder version of this is called twin-to-twin transfusion syndrome (TTTS). Acardiac twins experience a more extreme form of TTTS that may result in developmental issues for the foetuses.

Sesquizygotic Or Semi-Identical Twins

There have only been two cases of sesquizygotic twins, so this type is extremely rare.

In semi-identical twins, two separate sperm fertilise one egg. The fertilised egg then splits in two. Semi-identical twins share 100 per cent of the chromosomes from their mother but only about 50 per cent from the biological father as they each were fertilised by a different sperm in theory.

The truth is that scientists do not have the foggiest notion. While research is always being undertaken, given this condition is so specifically rare, everything is theorised and can be considered an ‘educated best guess’ scenario.

Female And Male Identical Twins

Sometimes identical twins can be assigned the sex of male and female at the point the egg divides. These twins start as identical males with XY sex chromosomes. But shortly after the egg divides, a genetic mutation called Turner syndrome occurs, leaving one twin with the chromosomes X0. Mother nature does what it is biologically programmed to do and makes the 0 an X. 


This twin will be assigned the gender of female at birth. Females of identical twins of the opposite sex can develop abnormally high testosterone levels and wide chests that lead to difficulties with hormone regulation, infertility, or fertility issues. The mutation does not affect the male twin.

Unique Fraternal Twins

Twins of different ages. Once a woman becomes pregnant, their hormone balances and compositions change, sending signals to the ovaries to stop releasing new eggs for potential fertilisation. There are always exceptions to the norm, and in rare cases, a phenomenon known as superfetation can occur. 

Superfecundation is the fertilisation of two or more ova from the same cycle by sperm from separate acts of sexual intercourse, which can lead to twin babies when a second egg is released and fertilised after a person has already conceived and is technically pregnant. It is known as superfecundation when this happens twice within one menstrual cycle.

In this case, both fertilised eggs will develop, but one twin will be slightly older than the other.


Twins With Different Fathers

If two or more eggs are released within a single menstrual cycle, each egg can be fertilised by sperm from a different father.

This is known as heteropaternal superfecundation. While this condition is common in some animals, it is considered rare in human women. There are circumstances where medical factors of a non-natural means can cause a spontaneous release of multiple ova, such as in fertility treatments designed to increase the number of eggs the ovary will release to increase the chance of conception in women with low fertility levels.

Polycystic ovaries, Mother Nature with a plan, even cysts or tumours on the ovary/s can sometimes cause the release of multiple eggs from the same ovary or cause both ovaries to release eggs at different times in the same month.

Perimenopause in some women will increase fertility levels and the chances of conceiving twins due to multiple egg releases each month in the ovaries lead up to reproductive retirement called menopause. It can be viewed as a biological desperation to conceive a child at the last minute and throwing everything in the arsenal at its disposal.

Twins With Different Skin Colours

It’s unlikely but not impossible to have twins with different ethnicities and skin tones, and it can happen in four clinically recognised formats:

  • When parents have different complexions, one of their fraternal twins may naturally resemble one parent, while the other twin looks more like the other parent.
  • The two biological fathers may have different ethnicities and skin tones in the rare case of heteropaternal superfecundation. Each twin would then receive the biological father’s genetics and skin colour or a blending of both parents to create a shade slightly different to both parents.
  • When both parents are biracial, this usually results in twins looking biracial. Take, for example, the American-Vietnamese babies conceived during the Vietnam war. They have very American builds, heights and features with lighter Asian skin tones but still appear somewhat Asian in overall appearance.
  • Occasionally, a recessive gene might kick in for unknown reasons, and one twin may present more genetic material from their DNA ancestors than the twin whose recessive gene did not activate. This can lead to the twins appearing to be from different ethnic races. See the video link below for proof that Mother Nature does what Mother Nature wants to do! Video of black Nigerian parents who gave birth to a white baby with blue eyes and blonde hair: https://www.youtube.com/watch?v=K7Rsunl3N3A

Medical Risks During A Twin Pregnancy

Twin pregnancies often come with an increased risk of developing some medical conditions. These can include:

Gestational diabetes. In gestational diabetes, the birthing parent’s blood sugar is too high during pregnancy.

Prematurity. A premature baby is born before 37 weeks of pregnancy.

Gestational hypertension. In gestational hypertension, the birthing parent’s blood pressure is too high during pregnancy.

Preeclampsia. Preeclampsia is a complication of pregnancy. With preeclampsia, you might have high blood pressure, high levels of protein in the urine that indicate kidney damage (proteinuria), or other signs of organ damage. Preeclampsia usually begins after 20 weeks of pregnancy in women whose blood pressure had previously been in the standard range.

Placenta previa. In placenta previa, the placenta sits too low in the uterus, covering the cervix.

Placental abruption. With placental abruption, the placenta detaches from the uterine wall, causing bleeding, back pain, and abdominal tenderness.

Placenta accreta. The placenta attaches too deeply into the uterine wall when placenta accreta occurs.

Low birth weight. A baby with low birth weight is born with a weight that’s less than 5 pounds and 8 ounces.

Postpartum haemorrhage. When a postpartum haemorrhage occurs, the mother experiences heavy bleeding after delivery, often because of an undelivered placenta or the uterus not contracting properly.

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