There are two main biological types of twins: identical (monozygotic) and fraternal (dizygotic). But the full picture is more nuanced than that. When you factor in how twins share a placenta, how late a fertilized egg splits, and a handful of rare phenomena, the number of recognized twin types expands to at least seven or eight distinct categories. In the U.S., about 110,000 sets of twins were born in 2023, at a rate of roughly 31 per 1,000 births.
Identical and Fraternal: The Two Main Types
The most fundamental distinction comes down to how many eggs and sperm are involved at conception.
Identical twins, also called monozygotic twins, form when a single egg is fertilized by a single sperm and then splits into two embryos. Because they come from one fertilized egg, identical twins share virtually the same DNA and are nearly always the same sex. They account for roughly one-third of all twin pregnancies.
Fraternal twins, or dizygotic twins, happen when two separate eggs are fertilized by two different sperm during the same cycle. Genetically, they’re like any other siblings, sharing about 50% of their DNA. They can be the same sex or different sexes, and they don’t necessarily look alike. Fraternal twinning is more common and is influenced by factors like maternal age, family history, and fertility treatments.
Three Subtypes Based on Placenta and Sac
Beyond the identical-versus-fraternal distinction, twins are also classified by how they’re set up inside the womb. This matters because it directly affects the risk level of the pregnancy.
Dichorionic-diamniotic (DiDi): Each twin has its own placenta and its own amniotic sac. This is the lowest-risk arrangement. All fraternal twins are DiDi. Identical twins can also be DiDi if the fertilized egg splits within the first three days after conception, before implantation. On ultrasound, this shows up as a “lambda” or “twin peak” sign where the membranes meet.
Monochorionic-diamniotic (MoDi): The twins share a single placenta but each has a separate amniotic sac. This happens when the split occurs between days 4 and 8 after conception, during implantation. Because the twins share a placenta, their blood supplies are connected through shared blood vessels. This creates a risk of twin-to-twin transfusion syndrome, where one twin gets too much blood flow and the other too little. MoDi twins have higher rates of growth restriction, premature birth, and other complications compared to DiDi twins, and they require closer monitoring throughout pregnancy.
Monochorionic-monoamniotic (MoMo): The twins share both a placenta and an amniotic sac. This is the rarest and highest-risk arrangement, occurring when the split happens between days 9 and 12. Because both babies float freely in the same sac with no membrane between them, their umbilical cords can become tangled. This cord entanglement is essentially unavoidable in MoMo pregnancies and requires intensive surveillance.
Mirror Image Twins
When an identical twin embryo splits around day 5 of development, the result can be mirror image twins. These twins have features that are reversed, as if one is looking at a reflection of the other. One might be left-handed and the other right-handed, or their hair whorls may spiral in opposite directions. Birthmarks and dental patterns can appear on opposite sides. Mirror image twins aren’t a separate genetic category; they’re a subset of identical twins defined by when the split occurs. If the split happens closer to day 9, the twins may still show mirror features but share a single amniotic sac, raising the risk profile of the pregnancy.
Semi-Identical Twins
Semi-identical (sesquizygotic) twins fall between identical and fraternal on the genetic spectrum, and they are extraordinarily rare. Only two confirmed cases have been reported in medical literature. They form when a single egg is fertilized by two different sperm simultaneously, creating a cell with three sets of chromosomes instead of the normal two. The embryo then sheds one of the extra paternal copies as it divides, eventually splitting into two babies. The result: twins who share 100% of their mother’s DNA but only about half of their father’s DNA, making them more genetically similar than fraternal twins but not as identical as monozygotic twins.
Because semi-identical twins of the same sex would look a lot like ordinary identical or fraternal twins, researchers believe more cases likely exist but have gone undetected. Standard genetic testing wouldn’t catch them unless clinicians were specifically looking.
Conjoined Twins
Conjoined twins develop when the fertilized egg begins to split very late, around 13 to 15 days after conception, but the separation is incomplete. Where the bodies remain connected determines the type. About 75% of conjoined twins are joined at the chest wall or upper abdomen. In chest-joined (thoracopagus) cases, the babies face each other and often share parts of the heart, diaphragm, or liver. Around 23% are joined at the hips, buttocks, or lower body, and roughly 2% are joined at the head (craniopagus), which is the rarest form.
The connection point determines whether surgical separation is feasible. Twins joined at the upper abdomen through a bridge of liver tissue (omphalopagus) generally have the most straightforward anatomy for separation. Twins who share a fused pelvis or sacral spine present a much more complex surgical picture.
Twins With Different Fathers
Heteropaternal superfecundation occurs when a woman releases two eggs in one cycle and each is fertilized by sperm from a different man during separate acts of intercourse. The second fertilization typically happens within three to four days of the first, though the window could stretch up to 14 days. The resulting twins are fraternal, with the unique twist that they are genetic half-siblings rather than full siblings. This requires polyovulation (releasing more than one egg), which is the same prerequisite for any fraternal twin pregnancy. Cases are rarely documented because the twins simply look like ordinary fraternal twins unless paternity testing reveals the difference.
Polar Body Twins: Still Theoretical
Polar body twins are a hypothesized type that would fall somewhere between identical and fraternal in terms of genetic similarity. During egg maturation, the body produces small cells called polar bodies that are normally discarded. In theory, a polar body could survive, be fertilized by a separate sperm, and develop alongside the main egg. If this happened, the twins would share all of their mother’s DNA (like identical twins) but have different fathers’ contributions (like fraternal twins). The expected genetic correlation for polar body twins would be around 0.38 to 0.51, compared to 0.50 for fraternal twins and 1.0 for identical twins. No case has been conclusively proven, and detecting them would require analyzing very specific genetic markers near the centers of chromosomes. For now, polar body twins remain a theoretical possibility rather than a confirmed type.