The difference comes down to one egg or two. Identical twins form when a single fertilized egg splits into two embryos. Fraternal twins form when two separate eggs are each fertilized by a different sperm. That one biological distinction (one starting cell or two) explains nearly everything that follows: why identical twins share virtually all their DNA while fraternal twins share about half, why identical twins are almost always the same sex while fraternal twins can be either, and why the two types of twin pregnancy are managed differently from the first trimester onward.
It’s one of the first questions people ask, and not just out of curiosity. Knowing which type you have matters for your pregnancy monitoring, for your children’s health history, and for questions that will come up years down the road.
How Each Type Forms
Think about what has to happen for twins to be identical. One fertilized egg. One set of chromosomes. And then, within the first two weeks after fertilization, that single cell splits into two. Both embryos carry the same genetic blueprint from the original cell, including the same sex chromosomes. That is why identical twins are virtually always the same sex, and why their DNA is as close to identical as two people’s can be. (The medical term is monozygotic, meaning “one zygote.”)
Identical twinning is less common than fraternal twinning. References vary, but roughly one-third of twin gestations are estimated to be identical; the exact proportion shifts over time because fraternal twinning rates change with maternal age and fertility treatment use. Most identical twinning appears to be sporadic with no clear cause, though rare familial clustering has been reported. One somewhat counterintuitive finding: assisted reproduction is associated with a higher rate of identical twinning, even when a single embryo is transferred.
Fraternal twins (dizygotic, meaning “two zygotes”) come from two eggs released during the same cycle, each fertilized by a different sperm. They develop as two genetically distinct individuals who happen to share a womb. Genetically, they’re full siblings: they share about 50% of their DNA on average, the same as any brother or sister born years apart. They can be the same sex or different sexes. They can look strikingly alike or nothing like each other.
Fraternal twinning does have a hereditary component. The tendency to release more than one egg during ovulation can run in families, which is why having a family history of fraternal twins on the mother’s side modestly increases the odds. Age, body size, and fertility treatment also affect the likelihood of fraternal twins. The full picture of what drives twin rates is more complicated than most people expect.
What “Identical” Actually Means Genetically
Identical is a useful shorthand, but it’s slightly misleading. Here’s what I mean: monozygotic twins start from the same DNA, but small differences accumulate over time. Some come from epigenetics: how genes are expressed can change without the underlying sequence changing. Others come from random mutations during cell division. By the time identical twins are adults, their genomes aren’t perfectly matched. But at birth, and for most clinical purposes throughout their lives, they are as genetically identical as two people can be.
Fraternal twins are no more closely related than any other pair of full siblings. If one fraternal twin develops a genetic condition, the other carries the same baseline risk as any other sibling, elevated compared to the general population if the condition has a heritable component, but not nearly as elevated as in identical twins.
This distinction matters for genetic counseling, cancer screening, and understanding inherited health conditions. If one identical twin is diagnosed with something that has a strong genetic driver, the other twin’s risk profile looks very different than if they were fraternal.
Why It Matters for Your Pregnancy
This surprised us, and it still surprises most newly diagnosed twin parents: the type of twins you’re carrying shapes your entire prenatal care plan.
What matters clinically is not zygosity (identical versus fraternal) but chorionicity, which refers to how many placentas the pregnancy involves. The two are related but they’re not the same thing.
Fraternal twins are almost always dichorionic, meaning two chorions and typically two separate placentas. In some cases those placentas can fuse and appear as one mass on ultrasound, which is part of why placenta count alone is not a reliable way to determine twin type. Identical twins may share a placenta or have separate ones, depending on when the original egg split. An early split gives each embryo time to form its own placenta; a later split results in a shared one.
When twins share a placenta (a monochorionic pregnancy), that creates a specific risk that dichorionic pregnancies don’t carry. Blood flows through a shared placenta via shared vessels, and those vessels can become unbalanced, causing one twin to receive more blood flow than the other. This condition, twin-to-twin transfusion syndrome, develops in roughly 10 to 15% of monochorionic pregnancies. Because of this, monochorionic twin pregnancies require ultrasound monitoring every two weeks starting around 16 weeks of gestation, compared to roughly every four weeks for dichorionic pregnancies.
If you’ve just been diagnosed with twins, one of the first questions your care team should answer is whether your twins are monochorionic or dichorionic. The best time to get that answer is before 14 weeks of gestation, when the structural markers are clearest on ultrasound. If no one has mentioned chorionicity yet, ask directly at your next appointment.
How to Find Out Whether Your Twins Are Identical or Fraternal
The short version: ultrasound can rule it in for identical twins, and sex can rule it in for fraternal twins, but for same-sex twins with two placentas, only a DNA test gives a definitive answer.
How doctors actually determine twin type involves a sequence. First-trimester ultrasound looks for specific structural markers: the lambda (twin-peak) sign indicates dichorionicity, the T sign indicates monochorionicity. A shared chorion strongly suggests identical twins, though rare exceptions, particularly after assisted reproduction, have been reported. Dichorionicity leaves the question open. If the anatomy scan shows opposite-sex twins, the question is settled: opposite-sex twins are almost always fraternal. If the twins are the same sex and have two placentas, they remain genuinely ambiguous until a DNA test is done.
This is where many families end up with the wrong answer. Same-sex twins with two placentas are frequently labeled fraternal at birth based on the placenta finding alone. But about 20 to 35% of identical twins are dichorionic. They split early enough to each develop a separate placenta, and those twins look identical to fraternal twins on every prenatal scan. Some of them spend their whole lives being called fraternal when they’re actually identical. I’ve met a few of them in twin parent communities.
A cheek-swab DNA test can resolve this definitively at any age. Most commercial kits require no blood draw or doctor visit, and with enough genetic markers, accuracy typically exceeds 99%. If you have same-sex twins who were called fraternal at birth based on the placenta finding and have never been DNA tested, that determination was a reasonable inference, not a confirmed fact.
After They’re Born
The biology doesn’t change, but what you know about it might. Many twin families leave the hospital with a label, identical or fraternal, that was assigned based on ultrasound findings and visual observation rather than genetic testing. For opposite-sex twins, that label is reliable. For same-sex twins, especially those born dichorionic, it may not be.
Physical appearance isn’t a reliable guide. Fraternal twins can look strikingly similar, especially as newborns. Identical twins can diverge in appearance over time as environment, nutrition, and experience shape them differently. Looking alike doesn’t confirm identical. Looking different doesn’t confirm fraternal.
If the question has medical relevance in your family (one twin has a heritable condition, you’re considering organ donation compatibility, or you’re doing genetic counseling), a DNA test is worth doing. If you simply want to know, that’s reason enough too.
What People Get Wrong About “Identical”
Two placentas means fraternal: it’s a reasonable shorthand, and it’s usually right (most di-di twins are fraternal). But clinicians sometimes present it as certainty when it isn’t, and that’s where same-sex di-di twins get mislabeled.
People also tend to assume that identical twins are essentially copies of each other. They share a genetic starting point, but they are individuals shaped by their own experiences from conception forward. Environmental differences in the womb, epigenetic variation, and everything that happens after birth means two people, not one person twice.
What they do share is a biological connection that matters in specific, practical ways: for pregnancy management, for health history, and for questions that keep surfacing throughout their lives.
The chorionicity question is the most urgent one during pregnancy. If you’ve just been diagnosed and no one has raised it yet, ask at your next appointment. The zygosity question (whether your twins are genetically identical or fraternal) can be answered later, but it is worth answering. For same-sex twins who were never DNA tested, a cheek swab is a simple thing that closes a question that has a way of coming up for the rest of their lives.
Sources
- First-trimester ultrasound determination of chorionicity in twin gestations using the lambda sign: a systematic review and meta-analysis. European Journal of Obstetrics and Gynecology, 2016. PubMed 27180271
- ISUOG Practice Guidelines (updated): role of ultrasound in twin pregnancy. Ultrasound in Obstetrics and Gynecology, 2025. PMC11788470
- Society for Maternal-Fetal Medicine Special Statement: Updated checklists for management of monochorionic twin pregnancy. American Journal of Obstetrics and Gynecology, 2021. AJOG S0002-9378(20)30947-9
- Twin-to-Twin Transfusion Syndrome. Cleveland Clinic. my.clevelandclinic.org
- Diagnosing zygosity in infant twins: physical similarity, genotyping, and chorionicity. Twin Research and Human Genetics, 2004. PubMed 14965457
- Twin chorionicity and zygosity both vary with maternal age. Prenatal Diagnosis, 2021. PMC8916645
