Polar twins, more formally called sesquizygotic twins, are a rare type of twinning that falls between identical and fraternal. They share all of their mother’s DNA but only a fraction of their father’s, making them roughly three-quarters genetically identical rather than the usual 100 percent (identical) or roughly 50 percent (fraternal). The concept has been debated for decades, but only one case has been genetically confirmed in the published literature, reported in 2019 in the New England Journal of Medicine. The science behind polar twinning involves an unusual event during egg cell division, and it reshapes some basic assumptions about how twin types are categorized.
How Polar Twinning Is Thought to Work
When an egg cell matures, it undergoes two rounds of division. Each round pinches off a tiny packet of chromosomes called a polar body. Normally, polar bodies wither away and play no role in reproduction. In polar body twinning, the theory goes, one of these discarded packets survives and gets fertilized by a second sperm, while the egg itself is fertilized by a first sperm. Because the egg and the polar body both originated from the same mother cell, the resulting embryos carry identical copies of the mother’s genome. But because two separate sperm cells provided the paternal contribution, the twins share only part of their father’s DNA.
This is distinct from either of the two familiar twin types. Identical twins come from a single fertilized egg that splits, so they share virtually all their DNA. Fraternal twins come from two separate eggs fertilized by two separate sperm, sharing about half their DNA on average, like any siblings. Polar twins occupy a genetic middle ground: maternally identical, paternally mixed. Geneticists coined the term “sesquizygotic” to capture this in-between status, from the Latin sesqui- meaning “one and a half.”
The Confirmed Case
In 2019, researchers in Australia and the Netherlands published a case of a monochorionic twin pregnancy, meaning the twins shared a single placenta, where routine ultrasound revealed that one twin was male and the other female. Sharing a placenta almost always signals identical twins, so a boy-girl pair was immediately puzzling. Genetic testing of amniotic fluid showed that the twins were maternally identical but shared only about 78 percent of their paternal genome. The researchers described them as sesquizygotic, genetically situated between monozygotic and dizygotic twins.1PubMed. Molecular Support for Heterogonesis Resulting in Sesquizygotic Twinning
The leading explanation for this case is that a single egg was fertilized by two sperm simultaneously. In most species, an egg has chemical barriers to block extra sperm after the first one enters, but these barriers occasionally fail. When two sperm fertilize one egg, the result is usually a nonviable embryo with three sets of chromosomes instead of two. In this particular case, however, the cell appears to have divided its contents into three groups, two of which each ended up with the normal two chromosome sets and went on to develop as separate embryos sharing a single placenta. The twins were chimeric, meaning each twin carried a mixture of two genetically distinct cell lines. Both were healthy at birth, though one twin later required medical intervention related to ambiguous genital development.
Earlier Clues From the 1980s
The idea that a polar body could participate in twinning predates the 2019 case by decades. In 1981, a research team published findings in Science describing a malformed acardiac twin, a twin that develops without a functioning heart and depends entirely on blood flow from its co-twin. The researchers found that the acardiac twin had three sets of chromosomes rather than two, and its tissues contained two maternally derived chromosome sets along with both of the mother’s immune-system marker combinations. They concluded that the twins had arisen from two independent fertilization events: one involving a normal egg and one involving its first-division polar body, each fertilized by a different sperm.2PubMed. Genetic studies of an acardiac monster: evidence of polar body twinning in man
This case was unusual because the polar body embryo turned out to be triploid, meaning it had that extra set of chromosomes. Triploid embryos almost never survive, and indeed this one developed severely abnormally. But the case was significant as the first published genetic evidence that polar body fertilization could happen in humans at all. For years afterward, polar body twinning remained a theoretical curiosity, cited in genetics textbooks but never confirmed in a pair of otherwise normal, living twins. The 2019 sesquizygotic case changed that.
Why These Pregnancies Are Hard to Detect
One reason polar or sesquizygotic twinning seems so rare may be that it is difficult to identify. Prenatal ultrasound can determine whether twins share a placenta, and genetic testing can reveal sex discordance or chromosomal abnormalities. But unless there is an obvious red flag, like a boy-girl pair sharing a single placenta, clinicians have no routine reason to order the kind of detailed genotyping that would reveal sesquizygosity.
In the confirmed 2019 case, the initial clue came from a first-trimester ultrasound showing a monochorionic diamniotic pregnancy, two amniotic sacs within a single chorion (the outer membrane that forms the placenta). This arrangement is characteristic of identical twins and occurs in only a small fraction of all twin pregnancies. Other case reports have documented monochorionic diamniotic twins with discordant sex, including a case from China where one twin had male genitalia and the other female, identified on ultrasound.3Gynecology and Women’s Health Care. A Case of Monochorionic-Diamniotic Twin Pregnancy with Differing Phenotypic Sex Sex discordance in monochorionic twins can arise from mechanisms other than sesquizygosity, such as the loss of a sex chromosome in one twin early in development, so each case requires careful genetic workup to determine the underlying cause.4PubMed. Monochorionic-diamniotic twins discordant in gender from a naturally conceived pregnancy through postzygotic sex chromosome loss in a 47,XXY zygote
The bottom line for detection is that without sex discordance or some other visible anomaly, a sesquizygotic pair could easily be classified as ordinary identical twins and never investigated further. We genuinely do not know how often this happens.
Obstetric Risks of a Shared Placenta
Whatever their genetic origin, twins who share a placenta face specific medical risks that twins with separate placentas do not. The most serious is twin-to-twin transfusion syndrome (TTTS), a condition where blood flow between the twins through shared blood vessels in the placenta becomes unbalanced, leaving one twin with too much blood and the other with too little. TTTS develops in roughly 9 to 15 percent of diamniotic monochorionic pregnancies and can be life-threatening for both twins if untreated.5PubMed. Placental markers of twin-to-twin transfusion syndrome in diamniotic-monochorionic twins: A morphometric analysis of deep artery-to-vein anastomoses
Sesquizygotic twins, because they share a placenta by definition in the one confirmed case, would face the same risk. This is one of the practical reasons the distinction matters clinically: any monochorionic pregnancy requires closer monitoring than a dichorionic one, regardless of whether the twins are truly identical, sesquizygotic, or something else entirely. Frequent ultrasound surveillance, sometimes every two weeks in the second trimester, is standard practice for monochorionic pregnancies to watch for signs of TTTS.
Chimerism in the Twins Themselves
In the confirmed sesquizygotic case, both twins were chimeras. That means each twin’s body contained cells from two genetically distinct cell lines. One twin had a roughly even mix of the two cell populations across most tissues, while the other was more skewed. This chimerism is what caused the sex discordance: one twin ended up with enough cells carrying male chromosomes to develop as phenotypically male, while the other twin had a different ratio. In at least one twin, the mixed cell populations affected sexual development significantly enough to require surgical attention.
Chimerism is not unique to sesquizygotic twinning. It can arise from the fusion of two fraternal-twin embryos early in development, from cells exchanged between a mother and fetus during pregnancy, or from a blood transfusion or organ transplant. But the form seen in sesquizygotic twins is distinctive because both individuals carry the same two cell lines in different proportions. In fraternal twins, by contrast, each twin has its own single cell line (barring any exchange through a shared placenta).
What This Means for Paternity Testing and Forensics
Chimerism, including the kind that might arise from unusual twinning events, creates genuine headaches for DNA-based identification. Standard paternity tests compare a set of genetic markers between the alleged parent and the child. If the parent is a chimera carrying two genetically distinct cell populations, a cheek swab might pick up one cell line while the relevant reproductive cells carry the other. The test could then falsely exclude a biological parent.
This is not just theoretical. Published case reports describe paternity disputes in which the tested parent was initially excluded as the biological parent, only for follow-up testing to reveal that the parent was a chimera. One well-documented case highlighted that some traditional paternity tests resulting in negative outcomes may have been wrong because of undiagnosed chimerism in the alleged parent, and stressed the importance of follow-up testing using cells from different tissues.6PubMed Central. A case of chimerism-induced paternity confusion: what ART practitioners can do to prevent future calamity for families Another forensic investigation identified a woman as a partial or whole-body chimera during a routine paternity study, underscoring that chimerism can be a pitfall in forensic contexts.7Forensic Science International: Genetics Supplement Series. A case of chimerism in a paternity study
Beyond paternity, microchimeric cells, tiny populations of foreign cells that persist in the body long-term, can differentiate into various tissue types including brain, heart, bone, liver, and lung cells. Their presence means an individual can carry two or more distinct DNA profiles, which complicates forensic identification and even biological sex determination from tissue samples.8PubMed. Microchimerism: The mystery of multiple DNA and its implications in forensic sciences For sesquizygotic twins specifically, the chimerism is more extensive than typical microchimerism, potentially affecting large fractions of the body’s cells. If one of these twins were ever involved in a forensic investigation, the mixed genetic profile could lead to confusion unless the investigator knew to look for chimerism.
What Animal Research Shows About Polar Body Viability
One of the longstanding questions about polar body twinning was whether a polar body’s chromosomes are even capable of supporting normal development. After all, polar bodies are supposed to be cellular waste, tiny packets that the egg discards to shed extra chromosomes during maturation. Researchers have addressed this directly in mouse experiments. In one study, the contents of a live first polar body were injected into an egg cell that had its own nucleus removed, then fertilized by sperm injection. When the resulting embryos were transferred to surrogate mothers, 30 to 57 percent developed into fertile offspring. The researchers concluded that chromosomes ejected into the first polar body have the same genetic potential as those that remain in the egg.9Biology of Reproduction. The First Polar Body Can Be Used for the Production of Normal Offspring in Mice
This finding supports the theoretical basis for polar body twinning: there is nothing inherently wrong with polar body chromosomes. They are a perfectly good copy of the maternal genome. The barrier to polar body twinning in nature is not genetic quality but physical survival. Polar bodies are tiny, lack the cytoplasmic machinery and nutrient stores of a proper egg cell, and almost always disintegrate quickly. For natural polar body twinning to occur, the polar body would need to be fertilized and somehow gain access to enough cellular resources to develop, an event that appears to be extraordinarily rare.
Separate mouse research has also explored how conditions during fertilization affect polar body behavior. Experiments with different calcium concentrations during in vitro fertilization found that high calcium levels disrupted the extrusion of the second polar body in certain mouse strains, though the effect was strain-specific.10PubMed. Effects of high calcium levels on the disturbed extrusion of the second polar body during in vitro fertilization in C3H/He mouse substrains This kind of research hints that the cellular mechanics of polar body formation can be disrupted by environmental conditions, though it does not directly demonstrate polar body twinning in animals.
Could Assisted Reproduction Increase the Chances?
Fertility treatments involve manipulating eggs and sperm in ways that bypass some of the body’s natural safeguards, and researchers have long noted that fertilization abnormalities are more commonly encountered following in vitro fertilization. The introduction of techniques like intracytoplasmic sperm injection, where a single sperm is physically injected into an egg, has changed both the rate and types of fertilization errors observed.11PubMed. Fertilization abnormalities following human in vitro fertilization and intracytoplasmic sperm injection Whether assisted reproduction could increase the probability of sesquizygotic twinning specifically remains unclear, partly because we still have so few confirmed cases to study. The one confirmed sesquizygotic pregnancy was naturally conceived. But given that IVF creates conditions where eggs and sperm interact in non-standard ways, and that errors like double fertilization by two sperm do occur at measurable rates in laboratory settings, the possibility is not far-fetched.
Some researchers have also noted that laboratory embryo manipulation can blur the lines between twin types in other ways. Experiments attempting to split human embryos in vitro, a process called embryo twinning, have shown that chromosomally abnormal embryos with three pronuclei (exactly the kind of cell that results from double fertilization) can be divided and sometimes develop to the blastocyst stage.12PubMed Central. Attempts for Generation of Embryonic Stem Cells from Human Embryos Following In Vitro Embryo Twinning These experiments were not aimed at creating sesquizygotic twins, but they demonstrate that embryos with abnormal fertilization events can sometimes survive and develop further than you might expect.
How Sesquizygotic Twins Differ Over Time
Even identical twins, who start with the same DNA, become genetically and biologically distinguishable as they age. Research on monozygotic twins has shown that while they are nearly indistinguishable in their chemical DNA modifications early in life, older identical twins show striking differences in these modifications across their genomes, affecting which genes are turned on or off.13PubMed Central. Epigenetic differences arise during the lifetime of monozygotic twins These changes accumulate from differences in diet, environment, stress exposure, and random cellular events over a lifetime.
Sesquizygotic twins would experience this same drift, but they start from a more different baseline than identical twins do. Because they share only part of their paternal genome, they already have built-in genetic differences from conception. Layer on the chimerism, where each twin carries a different mixture of two cell populations, and the biological differences between the pair are substantial from the very beginning. Over time, environmental and epigenetic factors would push them even further apart. In practical terms, sesquizygotic twins might look strikingly similar in some features (those governed by their shared maternal DNA) while differing noticeably in others (those influenced by their partially different paternal contribution), creating a resemblance pattern that does not fit neatly into either the “identical” or “fraternal” mold.
The Naming Confusion
The term “polar twins” gets used casually to describe this phenomenon, but it is worth knowing that the terminology is a bit of a mess. “Polar body twinning” refers specifically to the theoretical mechanism where a polar body is fertilized alongside the egg. “Sesquizygotic twinning” refers to the genetic outcome, twins who are maternally identical and paternally partial, regardless of the exact cellular mechanism. The 2019 case was described as sesquizygotic, but the researchers favored a two-sperm-one-egg mechanism over a polar body mechanism. The 1981 case was described as polar body twinning because the evidence pointed to the polar body being separately fertilized.
In other words, the two proposed mechanisms for producing half-identical twins are different, but both yield a similar genetic result: maternal identity with partial paternal sharing. Popular accounts often lump them together under “polar twins” or “half-identical twins,” which is understandable but glosses over a real mechanistic distinction. For most non-specialists, what matters is the genetic outcome: twins who are more alike than fraternal but less alike than identical, occupying a category that most people never knew existed. Whether the underlying event involved a polar body or double fertilization of a single egg is a question that currently matters more to reproductive biologists than to anyone else.