Identical twins form when one fertilized egg develops into two embryos. They are also called monozygotic twins. Fraternal twins form when two separate eggs are fertilized by two different sperm during the same menstrual cycle. They are also called dizygotic twins.
This biological difference matters for more than appearance or genetics. Many of the personal factors commonly associated with a higher chance of twins, including hyperovulation, family history, maternal age and some fertility treatments, affect the likelihood of fraternal twins much more directly than spontaneous identical twinning.
| Feature | Identical Twins | Fraternal Twins |
|---|---|---|
| Medical term | Monozygotic (MZ) | Dizygotic (DZ) |
| How they form | One fertilized egg develops into two embryos | Two separate eggs are fertilized by different sperm |
| Number of eggs involved | One | Two |
| Genetic similarity | Extremely genetically similar because they originate from the same fertilized egg | On average, genetically similar to each other like other full siblings |
| Sex | Normally the same chromosomal sex | Can be the same or different sexes |
| Appearance | Often very similar, but not necessarily indistinguishable | May look very similar or quite different |
| Placenta | May have separate or shared placental structures depending on early development | Develop with separate chorions; placental masses may remain separate or appear fused |
| Amniotic sacs | May be separate or, less commonly, shared | Separate |
| Family history | Most spontaneous cases do not show a strong inherited pattern | Can be influenced by an inherited tendency toward hyperovulation |
| Maternal age | No comparable established natural age effect | Increasing maternal age can increase the likelihood of multiple ovulation |
| Fertility treatment | Can occur after assisted reproduction, and some ART settings may increase MZ twinning | Can increase substantially when treatment results in multiple eggs or more than one transferred embryo |
| Relative frequency | Less common overall | More common overall |
How Identical Twins Form
Identical twins begin with a single egg fertilized by a single sperm. This creates one zygote, the earliest stage of the developing embryo. Very early in development, the developing conceptus separates into two embryonic cell populations, producing two embryos from the same original fertilized egg.
Because both embryos originate from the same zygote, these twins are called monozygotic twins, often abbreviated as MZ twins.
A simple way to visualize the process is:
1 egg + 1 sperm → 1 zygote → separation into 2 embryos → identical twins
Why Does One Embryo Split?
For most naturally conceived identical twins, scientists still do not know exactly why the early embryo separates. MedlinePlus Genetics notes that most cases of monozygotic twinning are not explained by known genetic factors, although unusual families with more identical twins than expected have been reported.
This uncertainty is important when discussing a person’s chance of identical twins. Unlike fraternal twinning, there is no well-established natural mechanism such as inherited hyperovulation that allows most spontaneous identical twinning to be predicted from family history or other common personal factors.
How Fraternal Twins Form
Fraternal twins follow a different biological pathway. Instead of one fertilized egg separating, the ovaries release two eggs during the same menstrual cycle. Each egg can then be fertilized by a different sperm, producing two separate zygotes and two embryos.
These twins are called dizygotic twins, or DZ twins.
The process can be summarized as:
2 eggs + 2 sperm → 2 separately fertilized zygotes → 2 embryos → fraternal twins
Genetically, fraternal twins have the same type of relationship as other full siblings. They happen to develop during the same pregnancy, but they originate from different eggs and different sperm.
What Is Hyperovulation?
Hyperovulation means releasing more than one egg during a single menstrual cycle. It is the key biological mechanism behind naturally conceived fraternal twins.
This helps explain why several known twin-related factors affect fraternal twinning rather than identical twinning. If a factor makes the release of multiple eggs more likely, it can increase the opportunity for two eggs to be fertilized during the same cycle.
Family tendency, maternal age and some fertility medications can all be relevant to this multiple-ovulation pathway.
Which Factors Affect Fraternal vs Identical Twin Chances?
It is misleading to treat every factor associated with twins as though it increases identical and fraternal twins equally. The two types arise through different biological mechanisms, so their probability factors are also different.
| Factor | Effect on Identical Twins | Effect on Fraternal Twins | Why |
|---|---|---|---|
| Family history | Little established effect in most families | Can increase likelihood | An inherited tendency toward hyperovulation can make release of multiple eggs more likely |
| Hyperovulation | Not the mechanism | Directly relevant | Two available eggs are required for ordinary dizygotic twinning |
| Increasing maternal age | No comparable established natural effect | Associated with increased likelihood | Multiple egg release becomes more likely at some later reproductive ages |
| Previous pregnancies / higher parity | No strong established natural effect | Associated with higher DZ twinning rates in population research | The association is observed epidemiologically rather than being a simple guarantee for an individual |
| Ovulation induction or ovarian stimulation | Not the main mechanism, although MZ twinning can still occur | Can substantially increase risk when multiple eggs mature | More than one mature follicle can produce more than one fertilizable egg |
| IVF / assisted reproductive technology | MZ twinning can occur and may be somewhat more frequent in ART than in natural conception | Risk rises especially when more than one embryo is transferred | Multiple transferred embryos can implant separately, while a single embryo can also occasionally divide |
Family History and Genetics
Family history is much more relevant to fraternal twinning than to identical twinning. MedlinePlus Genetics explains that dizygotic twinning is much more likely to run in families and is thought to be linked partly to inherited influences on hyperovulation.
This does not mean there is a single “twin gene” that guarantees twins. Research into specific genes affecting human hyperovulation has produced mixed results, and twin probability remains a complex trait influenced by multiple biological and environmental factors.
For identical twins, most spontaneous cases appear to occur without a strong hereditary explanation. Rare families with unusually frequent monozygotic twinning have been reported, but this is not the typical pattern.
Maternal Age and Previous Pregnancies
Maternal age mainly matters for the chance of fraternal twins. The American College of Obstetricians and Gynecologists notes that people older than 35 are more likely to release two or more eggs during a menstrual cycle. When more than one egg is released, there is an opportunity for separate fertilization and dizygotic twinning.
Population research also associates greater parity, meaning having had more previous births, with a higher rate of dizygotic twinning. These are population-level associations, not guarantees that a particular person will conceive twins.
Neither maternal age nor previous pregnancies provide a similarly established natural explanation for most spontaneous identical twinning.
Fertility Treatment and IVF
Fertility treatment can increase the chance of twins through more than one pathway.
- Ovulation induction and ovarian stimulation can cause more than one follicle to mature. If multiple eggs are released and fertilized, the resulting twins are fraternal.
- Transfer of more than one embryo during IVF can result in two separately created embryos implanting, producing dizygotic twins.
- A single transferred embryo can occasionally divide, resulting in monozygotic twins even after single-embryo transfer.
The American Society for Reproductive Medicine therefore treats multiple gestation as a risk that varies according to the fertility treatment being used. Modern reproductive medicine increasingly favors strategies such as appropriate single-embryo transfer to reduce preventable multiple pregnancies.
The important distinction is that fertility treatment does not simply “cause fraternal twins.” It can strongly increase dizygotic twinning under circumstances involving multiple eggs or embryos, while monozygotic splitting can also occur after assisted reproduction.
How Genetically Similar Are Identical and Fraternal Twins?
The genetic difference between the two twin types follows directly from how they form.
Fraternal twins come from different eggs and different sperm, so they share, on average, about half of the DNA variation that differs among people, similar to other full siblings.
Identical twins originate from the same fertilized egg and therefore begin development with essentially the same inherited genome. This is why they are usually much more genetically similar to one another than fraternal twins are.
DNA and Genetic Similarity
Calling identical twins “genetically identical” is useful shorthand, but it should not be interpreted to mean that every cell in both twins must remain genetically identical throughout life.
After the original embryo separates, each twin continues developing independently. DNA changes known as postzygotic mutations can arise after separation. Whole-genome research in monozygotic twins has demonstrated that small genetic differences can therefore exist between twins who originated from the same fertilized egg.
For most everyday comparisons, however, identical twins remain extraordinarily genetically similar. The scientific nuance matters mainly because describing them as perfect genetic clones is unnecessarily absolute.
Sex, Appearance and Individual Differences
Identical twins normally have the same chromosomal sex because they originate from the same fertilized egg. Fraternal twins may be the same sex or different sexes because their two eggs are fertilized independently.
Appearance is also not a perfect test of zygosity. Identical twins may look remarkably alike, but differences can develop in height, weight, facial details, birthmarks, health and other traits.
Even fingerprints are not identical. Development inside the uterus exposes each fetus to slightly different local conditions, and after birth each twin continues to accumulate different environmental experiences. Differences in gene expression and other biological processes can therefore contribute to increasing individuality over time.
Placenta and Amniotic Sacs: Zygosity vs Chorionicity
One of the most important distinctions in twin biology is also one of the easiest to misunderstand:
Zygosity describes whether the twins originated from one fertilized egg or two. Chorionicity describes whether they have one or two chorions, the outer fetal membranes closely related to placental organization. Amnionicity describes whether they have one or two amniotic sacs.
These concepts are related, but they are not interchangeable. In particular, having two placentas or two amniotic sacs does not automatically mean that twins are fraternal.
| Twin Pregnancy Type | Chorion / Placenta | Amniotic Sacs | Possible Zygosity |
|---|---|---|---|
| Dichorionic diamniotic (DCDA) | Two chorions; usually two placental masses, which may appear fused | Two | Fraternal or identical |
| Monochorionic diamniotic (MCDA) | One chorion with a shared placenta | Two | Identical |
| Monochorionic monoamniotic (MCMA) | One chorion with a shared placenta | One shared sac | Identical |
Dichorionic Diamniotic Twins (DCDA)
DCDA twins have two chorions and two amniotic sacs. All ordinary fraternal twin pregnancies are dichorionic diamniotic because the embryos originated independently.
However, some identical twins are also DCDA. If the monozygotic developmental separation occurs early enough, each developing twin can establish its own chorion and amniotic sac.
This is why seeing two placental structures on ultrasound does not, by itself, prove that same-sex twins are fraternal. Placental masses can also lie close together and appear fused.
Monochorionic Diamniotic Twins (MCDA)
MCDA twins share a chorion and placenta but have separate amniotic sacs. Under the standard classification used by ACOG, these twins are identical.
The shared placenta is clinically important because vascular connections can develop between the twins’ circulations within that placenta.
Monochorionic Monoamniotic Twins (MCMA)
MCMA twins share both one chorion and one amniotic sac. They also share a placenta and are identical.
This arrangement is less common and has pregnancy-management implications beyond the scope of a basic identical-vs-fraternal comparison. The important point here is that the placental and amniotic arrangement describes the structure of the pregnancy, while monozygotic status describes the twins’ biological origin.
Why Does Twin Classification Matter During Pregnancy?
During pregnancy, simply knowing that twins are “identical” or “fraternal” is not always the most clinically useful distinction. Chorionicity and amnionicity can be more important for determining surveillance and pregnancy care.
NICE specifically emphasizes that risks and clinical management differ according to chorionicity. A pregnancy in which twins share a placenta has different considerations from a pregnancy with separate placental circulations.
Shared Placenta and Pregnancy Monitoring
Monochorionic twins share a placenta and may have vascular connections between their circulations. One complication associated with shared placental circulation is twin-to-twin transfusion syndrome (TTTS), in which blood flow through the shared placental vessels becomes unbalanced between the twins.
The Society for Maternal-Fetal Medicine recommends early determination of chorionicity and specific ultrasound surveillance for monochorionic-diamniotic pregnancies because complications such as TTTS are related to shared placental circulation.
This does not mean that every identical twin pregnancy has the same risk. Some identical twins are DCDA and do not share a placenta. Pregnancy risk should therefore not be reduced to the simple statement that “identical twins are riskier than fraternal twins.”
Medical note: If you are pregnant with twins, your own ultrasound findings and your clinician’s assessment of chorionicity and amnionicity should guide pregnancy monitoring. General information about twin type cannot replace individual prenatal care.
How Can You Tell if Twins Are Identical or Fraternal?
Sometimes the answer is straightforward. In other cases, appearance or placental information alone cannot determine zygosity with certainty.
- Twins of different sexes are ordinarily fraternal.
- Monochorionic twins are classified as identical.
- Same-sex DCDA twins may be either identical or fraternal.
- Looking very similar does not prove twins are identical.
- Looking noticeably different does not prove twins are fraternal.
Ultrasound and Placental Clues
Ultrasound is important for determining chorionicity and amnionicity during pregnancy. Clinicians assess features such as the number of placental masses, the membranes separating the fetuses and characteristic ultrasound signs.
NICE recommends determining chorionicity and amnionicity in the first trimester because this information affects subsequent pregnancy care.
Ultrasound can sometimes also make zygosity clear. For example, monochorionic twins are monozygotic. But same-sex twins with a DCDA pregnancy may still be either monozygotic or dizygotic, so separate placentas cannot reliably settle every identical-vs-fraternal question.
DNA Zygosity Testing
When same-sex twins remain uncertain after birth, DNA zygosity testing provides the clearest way to determine whether they are monozygotic or dizygotic.
These tests typically compare DNA collected using cheek swabs from both twins. The American Academy of Pediatrics’ HealthyChildren resource recommends DNA testing when families want a definitive answer and explains that placental appearance, membranes and physical resemblance can sometimes be misleading.
Which Is More Common: Identical or Fraternal Twins?
Fraternal twins are more common overall.
MedlinePlus Genetics reports that monozygotic twins occur at roughly 3 to 4 per 1,000 births worldwide. Dizygotic twins are about twice as common overall, although their frequency varies much more between populations and according to maternal and reproductive factors.
This difference fits the underlying biology. The spontaneous rate of identical twinning is relatively stable, whereas the chance of fraternal twinning can change with factors that influence multiple ovulation and with some forms of fertility treatment.
What Does This Mean for Your Chance of Having Twins?
The most important practical takeaway is that there is not one single biological pathway called the “chance of twins.”
Fraternal twin probability can be influenced by measurable factors such as hyperovulation, family tendency, maternal age, reproductive history and fertility treatment. These factors can change the likelihood that two eggs or two separately created embryos are available during the same conception cycle.
Identical twin probability is much less predictable in natural conception. Most spontaneous monozygotic twinning occurs without a clearly established inherited or personal cause, although assisted reproduction may alter the rate to some degree.
This distinction matters when interpreting any estimate of twin likelihood. A personal factor may meaningfully change the chance of fraternal twins without having the same effect on identical twins.
Any twin-probability estimate should therefore be understood as an estimate based on known population-level associations, not as a prediction of exactly what will happen in an individual conception.
Frequently Asked Questions
Can identical twins be a boy and a girl?
In the usual biological pathway, identical twins have the same chromosomal sex because they originate from the same fertilized egg. Boy-girl twin pairs are therefore ordinarily fraternal twins formed from two separately fertilized eggs.
Can fraternal twins look almost identical?
Yes. Fraternal twins can look very similar, especially when they are the same sex and share many inherited physical traits. Appearance alone cannot reliably determine whether twins are monozygotic or dizygotic.
Do identical twins always share a placenta?
No. Some identical twins are dichorionic diamniotic and develop with separate chorions and usually separate placental masses. Other identical twins share a placenta. The arrangement depends on early development.
Can twins with two placentas still be identical?
Yes. DCDA twins can be either identical or fraternal. This is one reason placental appearance alone cannot always determine zygosity in same-sex twins.
Do twins skip a generation?
There is no biological rule requiring twins to skip a generation. A family tendency toward hyperovulation can make fraternal twins cluster in families, and the way that tendency is inherited and expressed can create an apparent generation-skipping pattern.
Can ultrasound prove twins are identical?
Sometimes. If twins are monochorionic, their shared chorion supports monozygotic classification. However, same-sex DCDA twins can be either identical or fraternal. When zygosity remains uncertain, DNA testing can provide a definitive answer.
Which type of twins is more predictable?
Fraternal twinning is more influenced by measurable factors such as hyperovulation, maternal age, family tendency and fertility treatment. Spontaneous identical twinning is comparatively less predictable because the reason one early embryo separates is unknown in most cases.
Sources and Evidence
This page is based on guidance and evidence from authoritative medical and scientific sources, including:
- American College of Obstetricians and Gynecologists (ACOG), Multiple Pregnancy
- MedlinePlus Genetics, National Library of Medicine, Is the probability of having twins determined by genetics?
- National Institute for Health and Care Excellence (NICE), Twin and Triplet Pregnancy
- Society for Maternal-Fetal Medicine (SMFM), Consult Series #72: Twin-Twin Transfusion Syndrome and Twin Anemia-Polycythemia Sequence
- American Society for Reproductive Medicine (ASRM), Multiple Gestation Associated With Infertility Therapy
- American Academy of Pediatrics / HealthyChildren.org, Identical and Fraternal Twins: Common Questions Answered
- Peer-reviewed whole-genome research on postzygotic genetic differences in monozygotic twins
Scope: This page is intended for education and general understanding. It does not diagnose twin type, predict an individual pregnancy outcome or replace advice from an obstetric, fertility or genetics professional.