But there are many other, less common, twin subtypes. These include conjoined twins, superfetation, heteropaternal superfecundation, polar body twins, semi-identical twins, and mirror-image twins.
Twin childbearing may be on the decline in the United States, but there are still plenty of twins out there. According to the Centers for Disease Control and Prevention (CDC), 31.1 out of every 1,000 births in the U.S. in 2020 were twins. Here is what you need to know about the different types of twins.
Types of Twins
You’ve probably heard of the most common types of twins—identical (monozygotic) twins, where the fertilized egg splits and develops two babies with no genetic differences, and fraternal (dizygotic) twins, where two eggs are fertilized by two sperm and the babies are genetically unique. But there are many other, less common, twin subtypes. These include conjoined twins, superfetation, heteropaternal superfecundation, polar body twins, semi-identical twins, and mirror-image twins. Read on to learn more.
Monozygotic Twins
Monozygotic (identical) twins are conceived from one fertilized egg, which separates into two embryos and later becomes two babies. Because each baby comes from the same egg and sperm, their genetic material (known as chromosomes) is identical. They are assigned the same sex at birth and have the same physical features, including eye and hair color.
However, if you’ve ever met identical twins and thought you identified a slight difference between them, this may be down to environmental differences before birth, like their positioning and space in the uterus. Also, different environments during childhood and beyond, such as illness and lifestyle choices, can turn specific genes on or off thanks to chemicals called epigenetic marks. Having different genes turned on may result in identical twins looking and acting differently.
Dizygotic Twins
Dizygotic (fraternal) twins each come from their own egg (released by the birthing parent’s body at the same time) and their own sperm. Fraternal twins share the same percentage of chromosomes as individual siblings born at different times (around 50%, per the National Human Genome Research Institute), which explains why they don’t look identical and can be assigned different sexes at birth.
Fraternal twins are about twice as common as identical twins, and they are much more likely to run in families. Plus, females with a mother or sister who have had fraternal twins are about twice as likely to have fraternal twins themselves, compared to the general population.
Conjoined Twins
Conjoined twins are identical twins that don’t completely separate from each other because the fertilized ovum fails to fully divide. This means when the babies are born they’re physically connected to each other at certain parts of their bodies, such as the abdomen (omphalopagus twins) or the chest (thoracopagus twins). Conjoined twins may also share internal organs or limbs.
Many conjoined twins die before or shortly after birth, but some surviving conjoined twins can be separated via surgery. The success of this depends on where the twins are joined and the organs they share.
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According to the Children's Hospital of Philadelphia, one in every 50,000 to 60,000 births is of conjoined twins. Pregnancy with conjoined twins comes with a much higher risk of serious complications and the babies are more likely to be born prematurely. Due to their anatomy, conjoined twins require delivery by C-section.
Superfetation
In very rare cases, an egg is released from the ovaries when the person is already pregnant. This leads to twins that are conceived at different times, known as superfetation.
In 2020, English mom Rebecca Roberts, 39, delivered "twins" conceived three weeks apart. Roberts told Good Morning America that her doctors had trouble understanding what was going on because superfetation is so rare that nobody knows the number of times it has happened.
Research suggests that superfetation may only happen in one out of a few million pregnancies; and that there have been fewer than 10 cases worldwide. As well as females ovulating twice in the same cycle or ovulating while already pregnant, experts believe that superfetation could occur if a female has a double uterus.
Heteropaternal Superfecundation
While fraternal (dizygotic) twins are the result of hyperovulation (the release of multiple eggs in a single cycle), superfecundation occurs when the eggs are fertilized by sperm from separate occasions of sexual intercourse.
And if a female has sex with different male partners before ovulating, the twins could have different fathers—known as heteropaternal superfecundation. This process is common in animals like cats, dogs, and cows, but extremely rare in humans. Researchers have said that the chance could be one in around 13,000, but nobody knows for sure.
In 2020, a case of heteropaternal superfecundation was reported in Colombia after paternity testing established that twin boys had different fathers.
Polar Body Twins
When fraternal twins look incredibly similar, it could be down to another possible twin type, which scientists have named polar body twins.
Polar body twinning remains no more than a theory because there are no observed cases of polar body twins and no method to identify or confirm polar body twinning.
After the ovaries release an egg, it can split into two parts, the smaller of which is called a polar body. This part contains all the chromosomes necessary to connect with sperm to make a baby. But it doesn't typically contain enough fluid, or cytoplasm, to survive.
Researchers suggest that it's possible that a polar body could survive and be fertilized. At the same time, the larger part of the original egg is fertilized by different sperm, leading to polar body (or polar) twins.
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Polar twins would share the same chromosomes from their birthing parent, but get different chromosomes from their non-birthing parent because they’re created from one egg but two different sperm. Therefore, they may or may not be assigned the same sex at birth and may look very alike, but not completely identical.
Semi-Identical Twins
A new type of twinning, semi-identical twinning, was revealed in 2006 in a report published in the Journal of Human Genetics. It was based on an unidentified set of twins, described as somewhere between identical and fraternal—they were identical on the mother's side but shared only half of their father's genes.
Semi-identical twins may develop when two sperm fertilize a single egg to create a triploid, which then splits. This is different from identical (monozygotic) twins, which form when a single fertilized egg splits into two; and fraternal (dizygotic) twins, which come from two separate eggs fertilized by two different sperm.
Just as they sound, mirror twins, or mirror-image twins, are mirror images of each other, meaning their hair may naturally fall in opposite directions, their teeth may grow in on opposite sides of their mouths, and they may have birthmarks, freckles, or other distinguishing marks on the opposite side of their bodies. Also, they typically have different dominant hands.
This unofficial phenomenon is caused when an egg splits seven to 12 days after fertilization (unlike in a typical identical twin pregnancy, when it splits during its first week after fertilization). The extra time means the egg has had long enough to develop a right and a left side.
A Word From Verywell
Most twins are fraternal or identical, but other types exist—and scientists believe there may be more still, which have yet to be officially confirmed. Any twin pregnancy comes with risks of complications, so parents should make sure to receive good prenatal care and seek medical advice if they have any concerns.
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