Pregnancy in hermaphroditic organisms is widespread in nature, occurring routinely in thousands of invertebrate species and a handful of vertebrates. In humans, the picture is far more constrained but not impossible: medical literature documents over a dozen pregnancies in people with ovotesticular disorder of sex development (the current clinical term for what was historically called “true hermaphroditism”), though every recorded offspring has been male. The biological reality spans an enormous range, from snails and tapeworms that routinely fertilize their own eggs to the rare human cases that require careful surgical and reproductive management.
Human Pregnancy With Ovotesticular DSD
Ovotesticular DSD is a condition in which a person has both ovarian and testicular tissue, sometimes in separate gonads and sometimes within a single structure called an ovotestis. It is rare, and pregnancy in someone with this condition is rarer still. A 2008 review in Obstetrics and Gynecology identified 11 reported pregnancies in people with confirmed ovotesticular DSD, noting that all known fetuses had been male. The review described a case of spontaneous pregnancy in a person with a 46,XX karyotype who had previously had partial removal of an ovotestis, and suggested that removing testicular tissue may actually improve fertility by allowing the remaining ovarian tissue to function more normally.1Obstetrics and Gynecology. Pregnancy in True Hermaphrodites and All Male Offspring to Date
A more recent tally, published in 2017, found 14 individuals with ovotesticular DSD who had achieved pregnancy, producing 26 pregnancies and 20 healthy babies between them. Again, every baby born was male. In about half of these cases, pregnancy occurred after the person had undergone gonadectomy (removal of one gonad, typically the one containing more testicular tissue). In others, pregnancy happened without prior surgery. The ovarian tissue in these individuals was generally functional and capable of ovulation, while the testicular tissue tended to be immature, though a few cases showed evidence of active sperm production.2PubMed. Potential autofertility in true hermaphrodites
The fact that all documented offspring have been male is striking and not fully explained. The leading hypothesis is that it relates to the genetic makeup of these individuals, most of whom carry a 46,XX karyotype with trace amounts of Y-chromosome material. But the sample size is small enough that drawing firm conclusions is difficult.
Assisted Reproduction and Modern Case Reports
Pregnancy for people with ovotesticular DSD often involves significant physical barriers beyond hormonal ones. Many individuals have undergone genital surgery during childhood, which can result in a narrowed or shortened vagina, making intercourse painful or impossible. A 2023 case report from Japan described a 29-year-old woman with ovotesticular DSD who had undergone gonadectomy as a child and later had reconstructive surgery. She ovulated spontaneously, but sexual intercourse was not possible due to pain from her earlier surgical procedures. After five failed rounds of artificial insemination, she conceived through in vitro fertilization and delivered a healthy boy at 37 weeks by cesarean section.3Journal of Nippon Medical School. Successful in Vitro Fertilization Pregnancy and Delivery by an Infertile Woman with Ovotesticular Disorder of Sex Development
This was only the eighth reported case of a live birth from a person with ovotesticular DSD after gonadectomy, and the first achieved through IVF. It highlights how modern reproductive technology can offer options for people whose anatomy creates barriers to conception, even when their ovarian function is intact. In at least one other case, a woman with 46,XX ovotesticular DSD conceived spontaneously without any assisted reproduction at all.4PubMed. SRY-negative 46,XX testicular/ovotesticular DSD: Long-term outcomes and early blockade of gonadotropic axis
Could a Human Hermaphrodite Self-Fertilize?
This is the question many people are really asking, and the honest answer is that it has never been documented and is considered extremely unlikely. Even though some individuals with ovotesticular DSD have both ovarian and testicular tissue, and a few have been shown to produce both eggs and sperm, the anatomical plumbing is not set up for internal self-fertilization. Sperm and egg would need to meet inside the reproductive tract in a way that human anatomy simply does not support, even in the presence of both tissue types. The testicular tissue is usually immature and poorly functional, and even when some sperm production has been observed, it has not been in quantities or locations that would allow fertilization of the person’s own eggs.
The 2017 review that documented both ovulation and spermatogenesis in the same individuals used this evidence to argue that “true hermaphrodites with ovarian and testicular tissues are potentially autofertile,” but this was framed as a theoretical possibility, not a clinical observation.5PubMed. Potential autofertility in true hermaphrodites No pregnancy from self-fertilization has ever been confirmed in a human. In every documented case, conception involved a sperm donor, a partner, or assisted reproduction.
Self-Fertilization in the Animal Kingdom
Outside of mammals, self-fertilization is a different story. Simultaneous hermaphroditism, where an individual produces both eggs and sperm at the same time, is the norm in many invertebrate groups. Earthworms, land snails, many flatworms, and numerous marine organisms are all simultaneous hermaphrodites. Some of these species can and do fertilize their own eggs, though most prefer to mate with a partner when one is available.
Among vertebrates, self-fertilization is vanishingly rare. The best-studied example is the mangrove rivulus, a small fish found in coastal habitats from Florida to Brazil. Mangrove rivulus populations consist mostly of hermaphrodites that produce both eggs and sperm internally and fertilize themselves. No functional females have ever been found in the wild or in captivity. Males do exist, but their proportion varies wildly across populations, from zero in some locations to about a quarter of the population in others.6Oxford Academic (Genome Biology and Evolution). The Genome of the Self-Fertilizing Mangrove Rivulus Fish, Kryptolebias marmoratus: A Model for Studying Phenotypic Plasticity and Adaptations to Extreme Environments Where males are more common, there is correspondingly more outcrossing (mating between individuals), but selfing remains the default.
This fish has become something of a star in biology precisely because it breaks the rules. Self-fertilizing vertebrates are so unusual that the mangrove rivulus is used as a model organism for studying everything from genetics to environmental adaptation. Being able to produce genetically identical offspring through selfing makes it possible to study how environment and epigenetics shape traits independently of genetic variation.
The Cost of Selfing
If self-fertilization works, why don’t more species do it? The answer comes down to inbreeding. When an organism fertilizes itself, the offspring inherits two copies of every gene from the same parent, dramatically increasing the chance that harmful recessive mutations will be expressed. In species that normally outcross, the fitness cost of a single generation of selfing can be devastating.
Research on the tapeworm Schistocephalus solidus illustrates this vividly. After one generation of selfing, the offspring retained only about 9% of the fitness of outcrossed controls, a catastrophic drop driven mainly by problems in early development. A second generation of selfing showed some improvement, with the load of harmful mutations estimated to drop by roughly half, consistent with the idea that the most lethal recessive mutations get “purged” when they are expressed and selected against. But the overall fitness remained substantially lower than in outcrossed worms.7Evolution. Lifetime inbreeding depression, purging, and mating system evolution in a simultaneous hermaphrodite tapeworm
This trade-off shapes the reproductive strategies of hermaphrodites everywhere. Many species that can self-fertilize treat it as a backup plan, only resorting to it when no mates are available. The mangrove rivulus is unusual in having evolved mechanisms that allow it to tolerate the genetic costs of routine selfing, likely through a long evolutionary history of purging harmful mutations from its genome.
How Temperature and Epigenetics Shape Hermaphrodite Reproduction
In the mangrove rivulus, whether a developing embryo becomes a hermaphrodite or a male is not purely a matter of genes. Temperature during development plays a significant role, and the mechanism appears to involve epigenetic changes, specifically DNA methylation patterns that influence gene expression without altering the underlying DNA sequence. Researchers found a significant interaction between sexual identity, temperature, and methylation patterns when selfing lines of the fish were raised at different temperatures, and identified several genes that were methylated differently in males versus hermaphrodites.8PubMed Central. Epigenetic regulation of sex ratios may explain natural variation in self-fertilization rates
This finding matters beyond one unusual fish. It suggests that environmental conditions can shift the ratio of males to hermaphrodites in a population by altering epigenetic marks during embryonic development. In practical terms, warmer or cooler conditions could change how much outcrossing happens in a population by changing how many males are produced.
Detailed studies of embryonic development in the mangrove rivulus have traced the waves of epigenetic reprogramming that occur after self-fertilization. Immediately after fertilization, DNA methylation drops sharply to around 16% during early development, then climbs back to about 70% as the embryo matures and organs begin to form. The enzymes responsible for adding, removing, and reading methylation marks show specific activity patterns in the gonads and brain of adults, as well as during early embryonic stages.9PubMed Central. DNA methylation in adults and during development of the self-fertilizing mangrove rivulus, Kryptolebias marmoratus Additional work has shown that histone modification enzymes are also active during embryonic development, peaking during early stages and declining later, pointing to a multi-layered system of epigenetic control over development and reproduction.10PubMed. The Kdm/Kmt gene families in the self-fertilizing mangrove rivulus fish, Kryptolebias marmoratus, suggest involvement of histone methylation machinery in development and reproduction
Parasites and the Advantage of Mating With a Partner
One of the most compelling arguments for why most hermaphrodites still prefer to mate with a partner, even when they could self-fertilize, comes from the Red Queen hypothesis. The idea is that organisms are locked in an evolutionary arms race with their parasites: as the host evolves resistance, the parasite evolves ways around it, and vice versa. Genetic diversity, which comes from combining genes with a partner, is the host’s best weapon in this race. Self-fertilization produces genetically identical or near-identical offspring, creating a population that a single well-adapted parasite could sweep through.
This was tested experimentally using the nematode Caenorhabditis elegans, a species that can reproduce either by outcrossing or by selfing. When populations of these worms were forced to coevolve with a bacterial pathogen, the ones that could outcross significantly increased their rate of sexual reproduction. Populations that were restricted to obligate selfing were rapidly driven to extinction by the pathogen, while outcrossing populations persisted through reciprocal adaptation.11PubMed Central. Running with the Red Queen: host-parasite coevolution selects for biparental sex
This experiment is one of the clearest demonstrations that sexual reproduction with a partner provides a real survival advantage under parasite pressure, even when self-fertilization is an available option. It helps explain why the overwhelming majority of hermaphroditic species maintain mechanisms for outcrossing and often go to elaborate lengths to find mates.
Trade-Offs Between Male and Female Functions
Hermaphroditic organisms face a resource allocation problem that single-sex organisms do not: how much energy to invest in eggs versus sperm. This is not a static decision. Research on plants, which are frequently hermaphroditic (producing both pollen and ovules), has shown that increasing investment in one sexual function often comes at the expense of the other. Experimental evolution in populations of hermaphroditic plants found that lineages evolving toward greater male function (more pollen production) showed corresponding decreases in female function (smaller flower biomass), revealing a clear genetic trade-off between the two.12Proceedings of the National Academy of Sciences. Sex-allocation trade-offs and their genetic architecture revealed by experimental evolution
For animal hermaphrodites, similar trade-offs play out in different ways. Many simultaneous hermaphrodites prefer to act as the male during mating, because sperm is cheaper to produce than eggs. When two individuals that both prefer the male role encounter each other, you get what biologists call a “gender conflict.” Various models have tried to explain how hermaphrodites resolve this conflict, including sperm trading (I’ll donate sperm to you if you donate to me), but no single framework fully accounts for the diversity of mating strategies observed across hermaphroditic species.13Elsevier. Reviews Sex role preferences, gender conflict and sperm trading in simultaneous hermaphrodites: a new framework
Viviparity in Hermaphroditic Invertebrates
When people hear “hermaphrodite pregnancy,” they often picture a mammal-like pregnancy with internal gestation. Among invertebrates, internal development of offspring (viviparity) does occur in some hermaphroditic species, though it looks nothing like mammalian pregnancy. Certain sea stars, for instance, brood their developing young internally within the gonad itself. In the viviparous sea star Cryptasterina hystera, researchers found that the switch to internal brooding did not require major changes to gonad structure, suggesting that the gonad was essentially pre-adapted to serve a marsupial-like function with relatively few anatomical modifications.14PubMed. Viviparity in the sea star Cryptasterina hystera (Asterinidae)–conserved and modified features in reproduction and development
This is a useful reminder that “pregnancy” in the biological sense, meaning the internal gestation of offspring, takes dramatically different forms across the tree of life. The elaborate immune tolerance, placental development, and hormonal cascades that characterize mammalian pregnancy are specific to mammals. In hermaphroditic invertebrates, internal development can be as simple as retaining fertilized eggs inside a body cavity until they hatch.
Ethical Dimensions of Intersex Reproduction
For humans born with ovotesticular DSD or other intersex conditions, questions about fertility and pregnancy are deeply personal and carry significant ethical weight. Historically, many children with ambiguous genitalia underwent surgical “correction” in infancy or early childhood, often including removal of gonadal tissue. When an ovotestis was removed, the potential for future fertility from that tissue was lost permanently, frequently before the child could participate in the decision.
Bioethicists have increasingly questioned these practices, and a related debate has emerged around preimplantation genetic diagnosis (PGD) for intersex conditions. PGD allows embryos created through IVF to be screened for genetic conditions before implantation, and it is technically possible to screen for some conditions associated with intersex development. An analysis in the American Journal of Bioethics argued that PGD to prevent conditions involving serious medical risks may be morally permissible, but that screening out “cosmetic” variations in sexual anatomy raises troubling questions about the broader implications for attitudes toward human diversity.15PubMed. Gender eugenics? The ethics of PGD for intersex conditions
The case reports of successful pregnancies in people with ovotesticular DSD carry their own ethical significance. They demonstrate that preserving ovarian tissue and offering reproductive support can lead to healthy pregnancies and births, adding weight to the argument that irreversible gonadal surgery in childhood deserves very careful scrutiny. Every case of a person with this condition who wanted to become pregnant and succeeded is, in a sense, evidence that the older surgical approach may have unnecessarily foreclosed reproductive futures.