Fraternal twins develop from two separate eggs fertilized by two separate sperm, making them genetically no more alike than any siblings born years apart. Identical twins develop from a single fertilized egg that splits into two embryos, giving them virtually the same DNA. That single distinction in how conception happens cascades into differences in genetics, physical resemblance, pregnancy risks, and even how useful each type of twin is to medical research.
How Each Type Forms
Fraternal twins, known in medicine as dizygotic twins, begin when a woman releases two eggs during the same cycle and both get fertilized. Each embryo implants separately in the uterus, each builds its own placenta, and each develops inside its own amniotic sac. Because two different sperm are involved, the twins can be any combination of sexes and share roughly half their DNA on average, just like siblings born in different years.
Identical twins, or monozygotic twins, start from one egg and one sperm. At some point in the first two weeks after fertilization, the developing embryo splits into two. What triggers that split remains surprisingly unclear, though researchers have studied the cellular mechanics in detail, particularly using data from assisted reproduction, where identical twinning is observed more frequently than in natural conceptions. The timing of the split determines how much the twins share in the womb. An early split (within the first few days) produces two separate placentas and two amniotic sacs. A later split means the twins share a single placenta, and in rare cases they share even the amniotic sac.
The Placenta Question and Why It Matters
One of the most medically important differences between fraternal and identical twins is what happens with the placenta. Fraternal twins almost always have their own individual placentas, a setup called dichorionic. Identical twins can be either dichorionic (two placentas) or monochorionic (one shared placenta), depending on when the embryo split. If splitting happens before the outer cell layer commits to forming the placenta, the twins get separate placentas. If it happens after, they share one.
This is not just an anatomical curiosity. Sharing a placenta introduces risks that twins with separate placentas do not face. A systematic review of outcomes in twin pregnancies found that monochorionic pregnancies consistently had worse results for the babies, including earlier delivery, lower birth weight, and higher rates of death around birth, compared with dichorionic pregnancies.1PubMed Central. Obstetrical and Neonatal Outcomes in Twin Pregnancies Based on Chorionicity: A Systematic Review of ART-Conceived Monochorionic vs. Dichorionic Twins Interestingly, the same review found that maternal complications like high blood pressure and gestational diabetes did not follow a consistent pattern by placenta type, suggesting the shared-placenta risk falls more heavily on the fetuses than on the mother.
The most dramatic complication unique to a shared placenta is twin-to-twin transfusion syndrome, where blood flow between the twins through shared vessels in the placenta becomes unbalanced. One twin ends up with too much blood volume and the other too little. It is estimated to affect roughly one to three in every 10,000 births.2PubMed Central. Overview and Long-term Outcomes of Patients Born With Twin-to-Twin Transfusion Syndrome This condition is impossible in fraternal twins because they never share a placenta.
How Genetically Alike Are Identical Twins, Really?
The popular version says identical twins have the same DNA, and for practical purposes that is close enough. But “virtually identical” is more accurate than “identical.” From the moment the embryo splits into two, each twin starts accumulating tiny genetic differences through random mutations that occur during normal cell division. These postzygotic mutations tend to land in non-coding regions of the genome and could influence gene expression, though most are harmless.3DNA Research. Functional landscape of genome-wide postzygotic somatic mutations between monozygotic twins
Beyond point mutations, identical twins can also differ in something called copy-number variation, where stretches of DNA are duplicated or deleted. A study that examined 19 pairs of identical twins found these copy-number differences existed in both twins who looked and acted alike and in twins who showed clear physical or medical differences.4American Journal of Human Genetics. Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy-Number-Variation Profiles The discovery was significant because it challenged the assumption that when identical twins develop different diseases, the explanation must be purely environmental. Sometimes the explanation is genetic, just not inherited in the usual sense.
A small number of identical twins also show evidence of microchimerism, where one twin carries trace amounts of the other’s cells. In one study, this was detected in about 8% of monozygotic twin pairs.5Europe PMC. Microchimerism in twins This cell swapping likely happens during fetal development when the twins share such close quarters.
Why Identical Twins Grow Less Alike Over Time
Even if you set aside the small DNA differences, identical twins diverge throughout life in ways that fraternal twins do not replicate. The mechanism is epigenetic: chemical tags on DNA and its packaging proteins that control which genes are active without changing the underlying code. A landmark study found that young identical twins are essentially indistinguishable in their epigenetic profiles, but older twins show striking differences in DNA methylation and histone acetylation patterns, which in turn alter which genes are switched on or off.6PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins
This divergence is not a one-time event. Longitudinal data shows that within-pair epigenetic differences grow steadily across decades, with some genomic regions accumulating discordance at a rate of 8 to 16 percent per decade.7PubMed Central. Epigenetic variation during the adult lifespan: cross-sectional and longitudinal data on monozygotic twin pairs This helps explain why one identical twin might develop an autoimmune disease or cancer while the other does not. Their DNA sequence is nearly the same, but the instructions for reading that DNA have drifted apart. Research on autoimmune diseases specifically has used identical twin pairs to show that environmental exposure can reshape epigenetic patterns and change disease susceptibility, even between people who started life with the same genetic hand.8Rheumatology Research. Genetic and epigenetic etiology of autoimmune diseases: lessons from twin studies
What Determines Whether You Have Fraternal Twins
The factors behind each type of twinning are very different. Fraternal twinning rates vary enormously across populations and are influenced by genetics, maternal age, and reproductive technology. Monozygotic twinning, by contrast, occurs at a remarkably stable rate of roughly 3.5 to 4 per 1,000 births worldwide, regardless of ethnicity or geography. The wide global variation in overall twinning rates is almost entirely driven by variation in fraternal twinning.9PLoS One. Twinning across the Developing World
Maternal age plays a well-documented role. As women age, declining ovarian reserve triggers a compensatory rise in follicle-stimulating hormone (FSH). That hormonal shift sometimes “overshoots,” causing more than one egg to mature and release in a single cycle. A study tracking nearly a thousand natural ovulatory cycles found that multi-follicle development was significantly more common in older women, with those women averaging higher basal FSH levels.10Human Reproduction. The paradox of declining fertility but increasing twinning rates with advancing maternal age This creates the paradox of declining fertility but rising twin rates as women get older.
Family history matters too, but only on the mother’s side and only for fraternal twins. A woman whose mother or sister had fraternal twins is more likely to release multiple eggs herself. The father’s family history of twins has no direct effect, since he does not control how many eggs are released. And because identical twinning is essentially random, family history does not predict it at all.
Fertility Treatments and Twinning
Assisted reproductive technology has reshaped the landscape of twinning over the past few decades. Fertility drugs that stimulate ovulation can cause multiple eggs to mature at once, directly increasing the chance of fraternal twins. In vitro fertilization historically involved transferring multiple embryos to improve the odds of pregnancy, which again raised fraternal twinning rates substantially. The trend in many clinics now is toward single-embryo transfer to reduce the risks that come with carrying multiples.
What is less intuitive is that assisted reproduction also increases the rate of identical twinning. A meta-analysis found that the rate of monozygotic twins after assisted conception was about 0.9%, compared with 0.4% in natural conceptions, meaning assisted reproduction roughly doubled the odds. Certain techniques carry even higher risk: blastocyst transfer was associated with over four times the risk, and intracytoplasmic sperm injection with about double the risk.11Human Reproduction Update. The risk of monozygotic twins after assisted reproductive technology: a systematic review and meta-analysis Why laboratory handling of embryos would encourage splitting is not fully understood, but it may relate to mechanical disruption of the embryo’s outer layer or changes in culture conditions.
Twin pregnancies conceived through IVF also carry distinct risks beyond those of spontaneous twins. An analysis of twin birth outcomes found that the IVF group had elevated risks of very preterm birth, birth defects, and placental complications compared with twins conceived without assistance.12PubMed Central. Adverse pregnancy, birth, and infant outcomes in twins: Effects of maternal fertility status and infant gender combinations
Fingerprints, Appearance, and Telling Twins Apart
Identical twins look remarkably alike, but they are not photocopies. Height, weight, and facial features are usually strikingly similar because those traits are strongly influenced by shared DNA. But fingerprints, which form partly in response to conditions in the womb like amniotic fluid pressure and fetal position, are unique to each twin. Automated fingerprint systems can distinguish identical twins without major drops in accuracy, though the twins’ prints are more similar to each other than to unrelated people. One study found that the chance of identical twins sharing the same fingerprint type on a given finger was about 74%, compared with roughly 32% for non-identical twins.13PubMed Central. Fingerprint recognition with identical twin fingerprints
Fraternal twins can look nothing alike, or they can closely resemble each other, the same way any two siblings might. Because they can be different sexes, a boy-girl pair of twins is a dead giveaway for fraternal. Same-sex fraternal twins, however, can be hard to distinguish from identical twins just by looking. When parents or doctors want certainty, DNA testing is the definitive method. Placenta examination at birth can also help: if there are clearly two separate placentas, the twins could still be identical (from an early split), but if there is a single shared placenta, they are almost certainly identical.
Sesquizygotic Twins and the Space Between
For a long time, the classification was simple: identical or fraternal, one egg or two. Then in 2019, researchers confirmed a case of sesquizygotic twins, a category that sits between the two. These twins resulted from a single egg that was fertilized by two sperm simultaneously. In this case, the twins were maternally identical but shared only about 78% of their father’s genome, making them genetically somewhere between identical and fraternal.14PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning The pregnancy was monochorionic (one placenta) but the twins were different sexes, a combination that is normally impossible because sharing a placenta implies a single fertilized egg, and a single egg should produce same-sex twins.
Sesquizygotic twinning appears to be extremely rare. The case was detected precisely because the sex discordance did not fit the usual rules and prompted deeper genetic testing. It is plausible that a small number of same-sex sesquizygotic twins have been misclassified as ordinary identical twins because nobody had reason to look further.
Vanishing Twins
Not every twin pregnancy ends with two babies. Vanishing twin syndrome refers to the spontaneous loss of one twin during the first trimester, and it is far more common than most people realize. Estimates put it at 15 to 35 percent of twin pregnancies.15PubMed. The vanishing twin: Diagnosis and implications The lost twin’s tissue is typically reabsorbed by the mother’s body or compressed into a small remnant alongside the surviving twin’s placenta. In many cases, the mother never knows a second embryo was present, especially if the loss occurs before the first ultrasound.
This phenomenon applies to both fraternal and identical twin pregnancies. Risk factors include the number of embryos transferred during IVF, the initial number of gestational sacs, and older maternal age. Because early ultrasound technology has improved, more vanishing twins are detected now than in previous decades, which also means more parents face the emotionally complex experience of learning they conceived twins but will deliver a singleton.
Why Twin Studies Matter to Science
The difference between fraternal and identical twins is not just a matter of pregnancy trivia. It forms the backbone of a major research method. Twin studies compare how similar identical twins are on some trait versus how similar fraternal twins are. Because identical twins share essentially all their DNA while fraternal twins share about half, any extra similarity in identical twins points toward a genetic contribution to the trait in question.16PubMed Central. Twin Studies: A Unique Epidemiological Tool
This logic has been applied to traits from personality and intelligence to blood pressure, psychiatric conditions, and lifespan. A Swedish study followed thousands of identical and fraternal twin pairs to estimate the genetic contribution to longevity.17The Journals of Gerontology: Series A. The Effect of Genetic Factors for Longevity: A Comparison of Identical and Fraternal Twins in the Swedish Twin Registry The most powerful version of these studies tracks identical twins who were raised apart, because any similarities between them cannot be chalked up to a shared household. Even so, generating reliable heritability estimates requires large sample sizes, and the assumptions underlying the classical twin method have their critics.18PubMed. Heritability Estimates from Twin Studies: The Efficiency of the MZA Design
The Evolutionary Puzzle of Fraternal Twinning
From an evolutionary standpoint, fraternal twinning is curious. Carrying two babies is riskier than carrying one, both for the mother and for each fetus. So why would natural selection preserve the tendency to release multiple eggs? One hypothesis holds that twinning is a costly byproduct of age-related hormonal changes and nothing more. A competing idea proposes that women who bear twins are not average mothers pushed beyond their limits but are instead unusually robust. A large historical analysis supported the second view, finding that mothers of twins had lower postmenopausal mortality, shorter intervals between births, later ages at last birth, and higher lifetime fertility than women who only bore singletons.19PubMed Central. Twinning in humans: maternal heterogeneity in reproduction and survival In other words, twinning may serve as a marker for a constitutionally stronger body rather than being a random accident of ovulation.
Identical twinning does not fit neatly into either evolutionary story. Because its rate is constant across populations and does not appear to be heritable, most researchers treat it as a developmental event without adaptive significance. No one has convincingly demonstrated that identical twinning benefits the mother or the offspring. It simply happens, at a steady low rate, for reasons the biology of early embryonic development has yet to fully explain.20Human Reproduction Update. Cellular mechanisms of monozygotic twinning: clues from assisted reproduction