How Rare Is Parthenogenesis in Humans?

No confirmed case of a human born entirely from parthenogenesis has ever been documented. The biological barriers are steep enough that most reproductive biologists consider a full human “virgin birth” effectively impossible under natural conditions. Yet the picture is more nuanced than a flat “it never happens.” Human eggs do sometimes begin dividing on their own, and a handful of living people carry a patchwork of parthenogenetic and normally fertilized cells in their bodies. The gap between “eggs can spontaneously activate” and “a baby can result” turns out to be enormous, filled with a specific evolutionary safeguard that mammals have spent millions of years reinforcing.

Why Mammalian Eggs Cannot Develop Alone

In many vertebrates, eggs occasionally develop without sperm. Parthenogenesis has been recorded in sharks, lizards, snakes, birds, and bony fish. A landmark case confirmed it genetically in a hammerhead shark, extending the list to nearly every major group of jawed vertebrates except mammals.¹ Mammals stand apart because of a mechanism called genomic imprinting.

Imprinting works like this: during the formation of sperm and eggs, certain genes get chemically tagged so that only the copy from one parent is active. Some genes are only “switched on” when inherited from the father; others only work when they come from the mother. A normally fertilized embryo gets one of each, and the system functions. A parthenogenetic embryo has two maternal copies and zero paternal copies, which means every gene that requires a paternal “on” switch is silent. That imbalance is lethal. Bi-parental reproduction is necessary in mammals precisely because the maternal and paternal genomes are not interchangeable after these epigenetic modifications take place during egg and sperm formation.²

When researchers have allowed parthenogenetically activated mouse eggs to develop in utero, the embryos consistently die around mid-gestation. The tissues that support the pregnancy, the placenta and yolk sac, are the first to fail, followed by collapse of the embryo’s own internal structures.³ This is not a quirk of one species. The same imprinting system operates across all placental mammals, humans included.⁴

One Theory for Why Imprinting Exists at All

There is an intriguing hypothesis about why mammals evolved imprinting in the first place. The “ovarian time bomb” hypothesis proposes that imprinting may have developed partly to prevent parthenogenetic embryos from forming inside the ovary, where they could become dangerous growths that threaten the mother’s life. Under this framework, imprinting is not just a side effect of parental genetic conflict but an active defense mechanism against spontaneous egg activation. The idea remains debated, but it fits neatly with the observation that when parthenogenetic events do occur in humans, they tend to produce tumors rather than embryos.

When Human Eggs Activate on Their Own

Human eggs are not inert. They sit in the ovary in a state of suspended animation, and their activation normally depends on a specific signal from sperm: a burst of calcium oscillations triggered by an enzyme the sperm delivers. But calcium fluctuations can sometimes happen without sperm. Researchers have shown that immature human eggs at a certain stage of development can produce spontaneous calcium oscillations on their own, which may reflect a degree of readiness to activate.⁵ These oscillations are normally part of the maturation process, but under certain conditions they can push the egg past the threshold into activation.⁶

When a human egg does spontaneously activate, it almost never develops into anything resembling an embryo. Instead, the most common outcome is an ovarian teratoma, a usually benign tumor that can contain a bizarre assortment of tissues: hair, teeth, bone, even fragments of thyroid or neural tissue. Teratomas are the remnants of an egg that started dividing and differentiating without any paternal genetic input. A recent review argued that human parthenogenesis is not as rare as traditionally reported; it simply manifests as tumor formation rather than embryonic development.⁷

On the paternal side, there is a mirror-image anomaly. A complete hydatidiform mole is a growth that results from an egg that loses its maternal genetic material and is “fertilized” by sperm whose genome then duplicates. The result is an entirely paternal genome with no maternal contribution. These moles develop placental tissue aggressively but form no fetal structures. They account for a small fraction of early pregnancy losses: one study found them in about a tenth of suspected molar pregnancies examined, which themselves represented a small share of early first-trimester losses.⁸ Together, teratomas and moles illustrate the same principle from opposite directions. An all-maternal genome makes disordered tissue that forms tumors. An all-paternal genome makes aggressive placental tissue that forms moles. Neither can build a person.⁹

Parthenogenetic Chimeras in Living People

The most striking cases in the medical literature involve people who are walking around with a mix of two genetically distinct cell lines, one parthenogenetic and one normally conceived. These individuals are called parthenogenetic chimeras, and though they are exceedingly rare, they are real.

The first genetically confirmed case was reported in 1995 in a boy who had developmental differences. Genetic testing revealed that some of his cells carried only maternal DNA, consistent with parthenogenetic origin, while others had normal biparental DNA. The researchers concluded that parthenogenetic chimerism can result in viable human offspring, though they called it a “presumably rare event.”¹⁰ The leading explanation is that an egg began dividing on its own, and then a separate fertilization event occurred, and the two cell populations merged into a single developing embryo. Because the normally fertilized cells supply the paternal imprinting the parthenogenetic cells lack, the combined organism can survive.

Another case involved a child with a mix of a normal cell line and a cell line carrying an extra chromosome 14. Detailed molecular analysis of dozens of genetic markers across tissues confirmed that the abnormal cells arose from parthenogenetic activation of the egg, which was then fertilized by two sperm. The result was a chimera whose two cell populations had different chromosomal makeups distributed across different tissues.¹¹

A more recent case, published in 2025, was discovered entirely by accident during a forensic investigation. A murder victim’s blood sample showed what appeared to be a DNA mixture from two people. When investigators tested 17 different tissue samples from the body, every single one showed the same mixed profile, with the ratio of the two components varying by tissue. The analysis revealed two complete cell lines, one 46,XX and one 46,XY, both sharing identical maternal chromosomes. The person had been an intersex parthenogenetic chimera their entire life without anyone knowing, with both cell lines contributing randomly to different organs.¹²

These cases are vanishingly rare in the published literature, with only a handful confirmed worldwide. But they raise an uncomfortable question: how many go undetected? Most people never have the kind of detailed multi-tissue genetic testing that would reveal chimerism. If the two cell lines are similar enough that the person looks and functions normally, there is no medical reason to investigate. The true frequency of parthenogenetic chimerism in the general population is unknown and likely unknowable with current screening practices.

What Scientists Have Achieved in the Lab

Researchers can artificially activate human eggs without sperm and coax them to develop for several days. Using chemical or electrical stimulation, unfertilized eggs can be pushed past the activation barrier, form a single pronucleus, begin dividing, and occasionally reach the blastocyst stage, the hollow ball of cells that would normally implant in the uterus. In one study, cryopreserved human eggs that were thawed and artificially activated showed an activation rate of about 86%, with roughly a sixth of cleaving embryos reaching blastocyst stage.¹³ Cryopreservation itself can trigger parthenogenetic activation in a fraction of eggs: around 27 to 29% of oocytes activated after freezing and thawing, compared with none among unfrozen controls.¹⁴

None of these lab-created parthenogenetic embryos could develop into a fetus. They are useful, though. Researchers have derived human embryonic stem cell lines from parthenogenetic blastocysts, generating cells that carry only maternal DNA.¹⁵ Highly homozygous stem cell lines have also been created from eggs that spontaneously activated during routine IVF, confirmed by the near-total absence of paternally expressed imprinted genes.¹⁶ These lines are valuable for research and potential therapies because their reduced genetic diversity makes them easier to match immunologically to donors.

In mice, scientists have gone much further. A 2018 study used gene editing to delete specific imprinted regions in egg-derived stem cells, then injected those edited cells into normal mouse eggs. The result was live “bimaternal” mice, born from two mothers with no paternal contribution. Some of these mice grew normally and were fertile. The same team also produced “bipaternal” mice from two fathers, though those pups died shortly after birth, requiring deletion of seven imprinted regions compared to three for the bimaternal mice.¹⁷ This experiment is the closest any mammal has come to genuine parthenogenesis, and it required extensive genetic engineering to get there.

A Related Puzzle in Fertility Clinics

In IVF, eggs occasionally fail to fertilize even when sperm is injected directly into them. When this happens repeatedly, clinics sometimes use calcium ionophore, a chemical that triggers the calcium oscillations an egg needs to activate. A meta-analysis of 14 studies found that adding calcium ionophore after sperm injection tripled both the clinical pregnancy rate and the live birth rate compared with standard injection alone.¹⁸ Safety data have been reassuring: an analysis of embryos created with this technique found no increase in chromosomal abnormalities and no significant differences in delivery timing or birth weight.¹⁹

The technique highlights an important point: the activation step is mechanically simple. Eggs are already primed to respond to a calcium signal, and all sperm really contributes at the activation stage is the right enzyme to start those oscillations. The complexity that prevents parthenogenesis is not about triggering cell division, it is about what happens after. Without paternal imprinting, the genetic program for building a placenta and a body simply does not run correctly.

Occasionally in IVF, an egg will activate entirely on its own and begin dividing without any sperm contribution at all. When this is detected through genetic testing, the resulting embryo shows a distinctive pattern: chromosomes inherited entirely from the mother, often with abnormal copy numbers. One case report described a woman whose embryos showed this pattern across multiple IVF cycles, with analysis confirming the hallmarks of parthenogenetic activation, including regions resembling uniparental disomy, where both copies of a chromosome come from the same parent.²⁰ Such embryos are not transferred because they cannot develop normally.

Uniparental Disomy and the Spectrum of Imprinting Errors

Parthenogenesis is the extreme case of a broader category of genetic anomalies involving imprinting. A less dramatic version is uniparental disomy, where a person inherits both copies of a single chromosome from one parent rather than one from each. The rest of their genome is normal, but that one chromosome pair carries two doses of whichever imprinted genes are silenced on the maternal or paternal copy. Depending on which chromosome is affected, this can cause specific syndromes. Two well-known examples involve chromosome 15: inheriting both copies from the mother can cause one condition, while inheriting both from the father causes a different one, because different imprinted genes are silenced in each direction.²¹ If having just one chromosome pair from a single parent can cause recognizable disease, it becomes clear why having every chromosome pair from a single parent, as in full parthenogenesis, is incompatible with survival.

How Courts and Patent Offices View Human Parthenotes

The question of whether a parthenogenetic embryo counts as a “human embryo” has real legal stakes. In 2014, the European Court of Justice ruled that human parthenogenetic stem cells fall outside the patenting prohibition that applies to procedures based on human embryonic stem cells. The reasoning was that because a parthenote cannot develop into a human being, it is not a human embryo under the European Biopatent Directive.²² This opened the door for companies to patent therapies derived from parthenogenetic stem cells without running afoul of embryo-protection rules.

The ruling rests on a biological fact: a parthenote’s developmental ceiling is real and well-documented. But the ethical boundary it draws is getting harder to maintain. As techniques for editing imprinted genes improve, the line between “cannot develop into a person” and “cannot develop into a person yet” grows thinner. The mouse experiments that produced live bimaternal offspring required deletion of only three imprinted regions. If that number shrinks further, or if the technique becomes feasible in human cells, the legal definition of a parthenote as a non-embryo may need revisiting. For now, though, the distinction holds: a human parthenote is a cellular entity that starts dividing but has no developmental future as a person, and regulatory frameworks treat it accordingly.