Twins that are not identical are called fraternal twins. The medical term is dizygotic twins, meaning they develop from two separate eggs fertilized by two separate sperm. Unlike identical twins, who split from a single fertilized egg, fraternal twins are genetically no more alike than any other pair of siblings. They just happen to share a womb at the same time.
How Fraternal Twins Form
Fraternal twinning starts with a process called hyperovulation, where a woman’s ovaries release two eggs in a single menstrual cycle instead of the usual one. Each egg is then fertilized independently by a different sperm cell. The result is two genetically distinct embryos developing side by side.
Because they come from separate eggs and sperm, fraternal twins share roughly 50% of their DNA on average, the same as any siblings born years apart. They can be the same sex or opposite sexes, and they often look no more alike than brothers and sisters typically do. Identical twins, by contrast, share nearly 100% of their DNA because they originate from one fertilized egg that splits early in development.
Fraternal twins are about twice as common as identical twins. In the United States in 2023, there were over 110,000 twin births total, at a rate of about 31 per 1,000 live births. The majority of those are fraternal.
What Increases the Chance of Fraternal Twins
Several factors make hyperovulation more likely. The mother’s age is one of the biggest: women over 30 (and especially over 35) are more likely to release multiple eggs per cycle due to hormonal changes. Body type matters too, with a higher BMI associated with increased twinning rates. The number of previous pregnancies, ethnic background, and diet also play a role.
Fraternal twinning runs in families, but pinning down exactly which genes are responsible has proven difficult. Researchers have looked for specific genetic variants linked to hyperovulation, and results have been mixed. What is clear is that the tendency passes through the mother’s side, since she is the one who ovulates. A father carrying “twin genes” wouldn’t affect whether his partner releases multiple eggs, though he could pass those genes to a daughter who might then have fraternal twins herself.
Fertility treatments have also had a major impact on fraternal twin rates. When multiple embryos are transferred during IVF, the chance of fraternal twins rises significantly. Even with elective single embryo transfer, where doctors place just one embryo, fraternal twins still occur in a small fraction of cases. An analysis of over 32,000 single embryo transfer cycles found that about one-fifth of all resulting twin pregnancies were fraternal, likely because a naturally released egg was fertilized alongside the transferred embryo.
How They Develop in the Womb
Fraternal twins almost always develop as dichorionic-diamniotic twins, meaning each baby has its own placenta and its own amniotic sac. This is the lowest-risk type of twin pregnancy because the two babies have completely separate blood supplies and no shared membranes. Identical twins can also be dichorionic-diamniotic if the egg splits early enough, so this arrangement alone doesn’t confirm whether twins are fraternal or identical. But if an ultrasound shows a single shared placenta, the twins are almost certainly identical.
Rare Variations Worth Knowing
Most twins fall neatly into either the fraternal or identical category, but biology occasionally produces something in between.
Semi-identical twins (called sesquizygotic twins) are extraordinarily rare. They occur when a single egg is fertilized by two different sperm before splitting. The result is twins who share all of their mother’s DNA but only a portion of their father’s, putting their genetic similarity somewhere between 50% and 100%. Only a handful of confirmed cases exist in the medical literature.
There’s also a phenomenon where fraternal twins have different fathers entirely. This happens through superfecundation: a woman releases two eggs and has intercourse with two different partners within the same fertile window, with each egg fertilized by sperm from a different man. The twins share their mother’s genetic contribution but have completely different paternal DNA. Case studies suggest this may be less rare than people assume, though it typically goes unnoticed unless paternity testing is done for other reasons.
Fraternal vs. Identical at a Glance
- Origin: Fraternal twins come from two eggs and two sperm. Identical twins come from one egg and one sperm that splits after fertilization.
- Shared DNA: Fraternal twins share about 50% of their DNA. Identical twins share close to 100%.
- Sex: Fraternal twins can be any combination of sexes. Identical twins are almost always the same sex.
- Appearance: Fraternal twins may or may not look alike. Identical twins typically look very similar, though not always perfectly matched.
- Frequency: Fraternal twins are roughly twice as common as identical twins and are influenced by genetics, maternal age, and fertility treatments. Identical twinning occurs at a relatively constant rate across populations.