When people think of identical twins, it's common to assume that these types of twins are virtual clones of each other. Many people believe that identical, or monozygotic, twins will act alike and look alike, and expect them to be identical in every way.
However, the term "identical twins" actually describes how the twins form, not what they look like. While the two babies form from the same genetic material, they develop independently. Additionally, genetic mutations can occur that create more differences between them. So, identical twins aren't always as identical as you might think.
What Are Identical Twins?
Identical twinning is officially described as monozygotic. Monozygotic twins form from a single (mono) fertilized egg (zygote). The zygote splits into two parts after conception, resulting in the development of two individual embryos. Because the two embryos are the result of a single egg/sperm combination, they have the same genetic origins and thus the same DNA.
Identical Twins and Zygosity
Dizygotic twins (commonly referred to as fraternal twins) are the result of two eggs fertilized by two separate sperm. Although most often only a single egg is released during each ovulation cycle, sometimes multipleeggs are released. When this happens, fraternal twins may occur. Dizygotic twins share about 50% of their genetic traits, the same as any other siblings born at different times.
All About Dizygotic Twins
Identical Twin Facts
Birth rates for identical twins are consistent across populations. They are the same regardless of race, geography, or maternal age. This is not true of fraternal twins.
Birth rate statistics for identical twinning have remained stable over the years, despite the overall increase in twins and multiples since the late 1980s. The odds of having identical twins is about 3 in 1,000, whereas the birth rate for all twins is about 33 in 1,000. So, identical twins are more unusual than fraternal twins.
Identical twinning when undergoing in vitro fertilization (IVF) is about twice as frequent as with natural conception. Identical twins represent about a third of all twins. Dizygotic twins are twice as common as monozygotic in assisted reproduction.
Identical twins are always the same sex (both have either two X chromosomes or one X and one Y). There is arare exceptionto this rule that involves a chromosomal defect.
Causes of Identical Twinning
The causes of monozygotic twinning are generally unknown. No one really understands why an egg splits. Technically, it's a malfunction of the normal development process.
There's no hereditary trait that makes it more likely for you to have identical twins. In other words, identical twins do not run in families. Although there are families with a high incidence of identical twins, it is due to chance or coincidence.
Despite their shared gene set, identical twins are unique individuals. No two individuals are exactly alike. They are influenced by slight differences in the environment in the womb as well as other factors once they are born.
A new type of twinning was identified in 2007. It occurs very rarely when two sperm fertilize a single egg, which then splits. Thus, the twins share the same DNA from the egg, but each gets a slightly different version of the DNA from the sperm.
Unique and Unusual Types of Twins
Sharing a Placenta
It was once believed that identical twins always shared one placenta. Even many doctors will mistakenly identify twins as fraternal when there are two placentas.
However, whether there are one or two placentas depends on when the egg splits. If it is early enough, the two embryos will implant separately in the uterus and develop individual placentas. If the split occurs later, they may share a placenta.
Twin pregnancies are more likely to have complications for both the fetuses and the pregnant person. More prenatal monitoring is common. When carrying multiples, the parent is at higher risk of pregnancy complications, such as gestational diabetes, anemia, preeclampsia, and miscarriage.
During pregnancy, twins are more likely to have growth problems, such as one twin being larger than the other (this is called discordant twins). Some types of monozygotic twins who share a placenta may be at risk for twin-to-twin transfusion syndrome. "Mo-mo" twins (monochorionic/monoamniotic) share a placenta and a single amniotic sac, and their umbilical cords may become entangled and compressed.
All multiple pregnancies are at greater risk of preterm birth, low birthweight, and neonatal death. Over 50% of twins are born prematurely (before 37 weeks). Premature babies are more likely to have health problems and need medical care in the newborn intensive care unit (NICU). Additionally, twins are about twice as likely as singletons to have birth defects or disabilities.
Risks of a Twin Pregnancy
How to Know If They're Identical
Identical twins are always the same sex (with extremely rare exceptions). So, if they are born boy-girl twins then you know they are fraternal. If they are the same sex, you can't necessarily tell by looking if twins are identical or fraternal.
Although many identical twins share a physical resemblance, so do many dizygotic (fraternal) twins. Clues to zygosity can be revealed in many ways, many of them relying on observations by the doctor, blood testing, or genetic testing.
Telling Identical Twins Apart
While many identical twins do look alike, they're not necessarily indistinguishable. Physical cues like hairstyle, moles, or freckles and their unique expressions or gestures provide clues totell twins apart.
Many people wonder whether fingerprinting can be used to distinguish between two twins who look remarkably alike. The answer, surprisingly, is yes. While their fingerprints will be similar, tiny differences in the womb environment may result in slightly different fingerprints.
8 Ways to Help People Tell Your Twins Apart
ESP and Secret Languages
Many people believe that identical twins share a special connection, including the ability to read each other's minds. Some also think twins can develop their own language with each other. Terms such as idioglossia, autonomous language, or cryptophasia describe the phenomenon of twin language, a concept that has intrigued researchers and parents alike. In fact, one study found that many identical twins shared a special language and 60% reported telepathic incidents.
Should Twins Be in the Same Class?
There is limited research on the pros and cons of keeping twins in the same school classroom. One large study showed no significant differences in achievement and other measures between twins kept in the same and different classes.
So, researchers speculate that there may not be a big advantage one way or the other. It can be a difficult decision, but parents should work with their schools to determine the optimal classroom placement for their twins.
Should Twins Have the Same Friends?
With a similar genetic background, many identical twins find that they have similar interests and preferences for establishing peer relationships. Often, they share many of the same friends, with more overlap than occurs with other types of twins. One older study found that elementary-aged identical twins shared about 50% of their friends, while fraternal twins only had about 25% of the same friends.
However, many identical twins also seek out different friends and enjoy establishing their unique identities through their friendship choices. Interestingly, research shows that a stronger twin relationship leads to better quality friend relationships with peers. Ultimately, whether twins have more or fewer of the same friends, they should be encouraged as individuals and given opportunities to develop their own identities and peer relationships.
Life Hacks for Parents of Twins or Multiples
Identical twins share the same genomes and are always of the same sex. In contrast, fraternal (dizygotic) twins result from the fertilization of two separate eggs with two different sperm during the same pregnancy. They share half of their genomes, just like any other siblings.How rare are identical twins? ›
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.Do identical twins share 100%? ›
They are identical because both siblings have the same set of DNA from mom and the same set of DNA from dad. Since both sets are the same they share 100% of their DNA. Fraternal twins are really just siblings who share the same womb. Each starts out as a separate egg fertilized by a separate sperm.Are identical twins a miracle? ›
“Identical twins are considered a 'miracle' of nature and we've not fully understood the biological processes that lead to their formation,” Deputy Director of Twins Research Australia, Professor Jeff Craig said.What is the rarest type of identical 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.
Twins not only have a bestie from birth — they also live longer than singletons. And those two factors may be related, according to new University of Washington research.Do identical twins have more problems? ›
While there are risks associated with all twin pregnancies, dizygotic (fraternal) twin pregnancies usually bring about the least amount of complications, carrying the lowest risk of all types of twins. Worse complications are possible with monozygotic (identical) twins because the embryo is dividing.How much DNA do twins share? ›
Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes.Do identical twins look the same forever? ›
As identical twins get older they may look more and more different, because they are exposed to more diverse environments. The science of epigenetics explains how these environmental influences can affect the genes.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).
Similar to the mind-reading myth, there are things that can't be explained. There are twins who say they have felt each other's pain. And their close relationship and nearly-identical physical structure means there could be a sliver of truth in the theory — as pain can be psychological and can be felt empathetically.Do twins have same fingerprints? ›
They come from the same fertilized egg and share the same genetic blueprint. To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.What is the hardest thing about having twins? ›
The hardest thing about having twins is...
“Managing the movement of two babies. Carrying them both up and down the stairs, getting them into the car, etc.” —Simeon R. “Often having to make one baby wait!” —Catharine D. “Being outnumbered—the logistics of two on one is definitely the hardest.
Identical twins have identical genes but they don't have identical brains and that is because learning leads to anatomical changes in the brain and even identical twins will have different social experiences, different learning experiences, and therefore will end up having different brains.What is the life expectancy of identical twins? ›
When analyzing the data by gender, the researchers found that female identical twins lived, on average, about 63.4 years, whereas female fraternal twins lived about 61.4 years and the general Danish female population lived about 58.8 years, Sharrow said.
The Science of Fraternal Twins. Mixed-gender twins are the most common type of fraternals, some 50 percent are boy-girl.How common are identical twins naturally? ›
Monozygotic (MZ) twins, also called identical twins, occur when a single egg cell is fertilized by a single sperm cell. The resulting zygote splits into two very early in development, leading to the formation of two separate embryos. MZ twins occur in 3 to 4 per 1,000 births worldwide.Do identical twins have 100% the same DNA? ›
The study of 381 pairs of identical twins and two sets of identical triplets found that only 38 were genetically identical, Tina Hesman Saey reports for Science News. Most had just a few points of genetic mismatch, but 39 had more than 100 differences in their DNA.Which twin lives longer? ›
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.Do mothers of twins live longer? ›
For women born 1870-1899, moms of twins averaged reproductive spans of 14 years 11 months versus 14 years for singleton moms. Both results were statistically significant. Moms of twins also were older at the time of their last birth.
Using the family-based adult sample, no differences in IQ scores were found between twins and their singleton siblings.Do identical twins have the same IQ? ›
Identical twins are definitely more likely to share the same IQ than are fraternal twins. So genes do play a role in IQ.Do identical twins have the same personality? ›
Thus identical twins, though they start with the same genes, likely develop different personalities in the same environment partially based on how they interact with their environment.Which parent carries the twins gene? ›
This is why fraternal twins run in families. However, only women ovulate. So, the mother's genes control this and the fathers don't. This is why having a background of twins in the family matters only if it is on the mother's side.Which parent determines twins? ›
For a given pregnancy, the odds of conceiving fraternal twins are only determined by the mother's genetics, not the father's.Can a twin 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.Do identical twins have the same birthmarks? ›
Twins might share the same DNA, but they almost certainly don't share the same moles, freckles, or birthmarks. It can be helpful to take pictures of each child's birthmarks so that if you do get to the point where you need to rely on birthmarks to tell them apart, you'll be able to.How likely is it to have twins again? ›
What is the likelihood of having more than one set of twins? If existing twins are identical the chance of having another set is the same as most women, about 1 in 250. If the twins are non- identical the chances of having twins again are much higher.Can twins have different birthdays? ›
For the most part, twins and multiples share the same birthday. However, depending on the time of day the babies are born and how long the timespan is between each baby's birth, twins can be born on different days.Do identical twins have separation anxiety? ›
Anxiety and depression about missing your twin are normal if you are a twin. The loneliness that twins experience is impossible for non-twins to understand.
- You Have A Built-In Best Friend. ...
- You'll Never Get A More Honest Opinion. ...
- You Have Back Up (All The Time) ...
- You Can Borrow Stuff Without A Row. ...
- You Can Dress The Same. ...
- You Can Freak People Out.
Everyone knows that twins have a special connection between them, and can sometimes posses a supernatural bond. A twin can sense when their sibling is in pain, and they sometimes "share a brain" by reacting to the same situation in an identical way.Do twins have a bond? ›
Identical twins share a particularly intense bond. They are the result of one fertilized egg splitting into two, giving them identical DNA. (Fraternal twins are the result of two separate, genetically different fertilized eggs.) As a result, identical twins are as close as two people can be.Do twins have the same DNA crime? ›
Because identical twins have the same DNA, it is nearly impossible to distinguish between these individuals when analyzing DNA for paternity testing or for evidence of a crime.Why do identical twins look different? ›
Some identical twins can have different heights and weights. This is because height and weight are controlled by what you eat as well your DNA. Differences in diet can start earlier than you might imagine. When the twins are growing inside the uterus, there can be differences in how well they connect to the placenta.What do twins struggle with? ›
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.What are the odd of having twins? ›
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.Why do identical twins end up having such different lives? ›
The causes of these differences were due to changes in the human epigenome, he realised. "Essentially, epigenetics is the mechanism by which environmental changes alter the behaviour of our genes," he says.Do identical twins have the same talents? ›
Identical twins have exactly the same DNA, but they are not exactly alike. Each twin has his or her own personality, talents, likes, and dislikes.Why do identical twins have different brains? ›
Identical twins have identical genes but they don't have identical brains and that is because learning leads to anatomical changes in the brain and even identical twins will have different social experiences, different learning experiences, and therefore will end up having different brains.