Twins can absolutely be different sexes, and in fact it happens all the time with fraternal twins. Roughly a third of all twin pairs born are boy-girl combinations, which arise whenever two separate eggs are fertilized independently. The more surprising question is whether identical twins can differ in sex, and the answer there is also yes, though it is exceptionally rare and involves unusual chromosomal events during early development. The biology behind both scenarios reveals how sex determination works at its most fundamental level and where it can take unexpected turns.
How Fraternal Twins End Up as Boy-Girl Pairs
Fraternal twins, also called dizygotic twins, develop from two eggs released during the same ovulation cycle and fertilized by two different sperm. Because each fertilization event is independent, each embryo gets its own random draw for sex chromosomes. A sperm carrying an X chromosome produces a girl (XX), and a sperm carrying a Y produces a boy (XY). The odds for each embryo are close to 50-50, so the chance of a fraternal twin pair being one boy and one girl is roughly the same as getting one head and one tail from two coin flips.
A classic method for estimating how many twins are fraternal versus identical, developed by the German physician Wilhelm Weinberg in 1901, relies on this logic. His “differential rule” reasons that opposite-sex twin pairs must be fraternal, and that the number of same-sex fraternal pairs should be about the same, since sex assignment is random for each embryo. Subtract same-sex fraternal pairs from all same-sex pairs, and you get an estimate of identical twins.1PubMed. Weinberg’s differential rule reconsidered The rule has held up reasonably well as a rough guide, though researchers have noted that its core assumptions, particularly that the probability of being male is exactly equal and independent for both embryos, may not hold perfectly in every population.2PubMed. The current status of Weinberg’s differential rule
Why Identical Twins Almost Always Share the Same Sex
Identical, or monozygotic, twins form when a single fertilized egg splits into two embryos. Because both embryos originate from one zygote with one set of chromosomes, they carry the same sex chromosomes. Two XX embryos will both develop as female; two XY embryos will both develop as male. For the vast majority of identical twin pairs, sex is a settled matter from the moment the egg is fertilized, long before it splits.
This is why doctors performing early ultrasounds on twins use sex discordance as a clue. If one twin appears male and the other female, the working assumption is that the pair is fraternal rather than identical. That assumption holds in nearly all cases, but “nearly all” is not the same as “always.”
Rare Chromosomal Events That Split Identical Twins by Sex
In a handful of documented cases, monozygotic twins have ended up phenotypically different sexes. The mechanism usually involves a mistake during cell division that happens around the same time the embryo splits in two. If the original zygote is XY and the Y chromosome is lost from some cells early in development, the result is mosaicism, meaning some cells carry XY and others carry only X (written as 45,X/46,XY). When the embryo then splits into twins, one twin can end up with a higher proportion of XY cells and develop as male, while the other inherits more 45,X cells and develops features consistent with Turner syndrome, a condition associated with having only one X chromosome.3PubMed. Monozygotic twins with 45,X/46,XY mosaicism discordant for phenotypic sex
One especially well-characterized case involved monozygotic twins with mosaicism involving an abnormal dicentric Y chromosome. The unequal distribution of cell lines between the two twins during the twinning event produced strikingly different outcomes: one twin appeared to be a normal boy, while the other presented as a girl with features of Turner syndrome.4American Journal of Medical Genetics. Monozygotic twins of discordant sex both with 45, X/46, X, idic(Y) mosaicism A separate report documented monozygotic twins who were mosaic for 47,XXY/46,XX cell lines. The twin who received a greater share of XXY cells developed as male with mild Klinefelter features, while the other twin, dominated by XX cells, developed as female.5PubMed. Discordant sex in monozygotic XXY/XX twins: a case report
These cases are vanishingly rare, amounting to scattered case reports in the medical literature rather than anything a prospective parent needs to worry about. But they reveal that sex determination is not simply “read the chromosomes.” The timing and distribution of chromosomal changes relative to the twinning event matters enormously.
Sesquizygotic Twins and the Space Between Identical and Fraternal
For most of the history of twin research, twins were neatly sorted into two bins: identical or fraternal. That binary was challenged by the discovery of sesquizygotic twins, sometimes called “semi-identical.” In this unusual scenario, a single egg is fertilized by two separate sperm before it divides. The resulting embryo contains three sets of chromosomes initially, but the cell sorts itself into two cell lines as it divides, each carrying a different paternal contribution. When the embryo then splits into twins, each twin can inherit a different mix of those cell lines.6Twin Research and Human Genetics. The New Sesquizygotic Twins and More
Sesquizygotic twins share all of their mother’s DNA, like identical twins, but share only a fraction of their father’s, like fraternal twins. If the two fertilizing sperm carried different sex chromosomes, one X and one Y, the twins could develop as different sexes while still being genetically closer than any ordinary fraternal pair. The first confirmed living case, reported in 2019, involved a boy-girl pair identified through genetic testing after doctors noticed the twins shared a placenta, a feature typically associated with identical twins. This type of twinning is thought to be extremely rare, and only a few cases have been confirmed using modern genomic methods.
Same Chromosomes, Different Bodies
Sex chromosomes set the stage for development, but they do not have the final say on their own. Hormones, hormone receptors, and the genes that regulate them carry out the actual work of sexual differentiation. This means twins can share identical sex chromosomes and still end up looking very different physically.
Complete androgen insensitivity syndrome, or CAIS, is one of the clearest examples. People with CAIS have XY chromosomes and produce testosterone, but their cells cannot respond to it because the androgen receptor does not function. The result is external anatomy that is typically female, often including breasts and a vagina, but with no uterus or ovaries. A case report described 46,XY twins, both carrying the same chromosomes, where one twin had CAIS and developed a female phenotype while the co-twin developed as a typical male.7PubMed Central. Complete androgen insensitivity syndrome in twins with discordant phenotypes: a case report and review of the literature In another case, monozygotic twins with CAIS were found to each carry two new mutations in the androgen receptor gene, mutations that arose spontaneously rather than being inherited.8The Journal of Clinical Endocrinology & Metabolism. Two de Novo Mutations in the AR Gene Cause the Complete Androgen Insensitivity Syndrome in a Pair of Monozygotic Twins
These cases complicate any straightforward equation of chromosomal sex with physical sex. Twins who are genetically identical can follow divergent paths of sexual development if the hormonal machinery breaks down differently in each individual.
How Epigenetics Creates Differences Between Identical Twins
Even when identical twins share the same DNA sequence down to the letter, their genes are not necessarily used in the same way. Chemical modifications to DNA and the proteins that package it, collectively called epigenetic marks, influence which genes get turned on or off in each cell. A landmark study of monozygotic twins found that while young twins were epigenetically almost indistinguishable, older twins showed large differences in DNA methylation and histone acetylation patterns across their genomes.9Proceedings of the National Academy of Sciences. Epigenetic differences arise during the lifetime of monozygotic twins
These epigenetic differences can affect gene expression broadly, contributing to discordance in everything from disease susceptibility to physical appearance.10PubMed Central. Epigenetics of discordant monozygotic twins: implications for disease While epigenetic changes alone would not switch an entire sex-determination pathway, they could amplify the effects of other mechanisms, like the uneven distribution of mosaic cell lines, to push otherwise-similar twins toward different developmental outcomes. The field is still working out exactly how much epigenetics contributes to sexual development discordance specifically, but the broader lesson is clear: sharing a genome does not guarantee sharing a phenotype.
Blood Chimerism in Opposite-Sex Twins
When fraternal twins of different sexes share a placenta, something unusual can happen. Blood vessels can form connections between the two fetal circulations, allowing cells from one twin to colonize the other. The result is blood chimerism, where a person’s blood contains cells with a different set of chromosomes from the rest of their body. In one documented case of a monochorionic dizygotic opposite-sex twin pregnancy conceived after fertility treatment, vascular anastomoses between the two fetal circulations were confirmed, and postnatal testing showed chimerism in the blood cells of both twins.11Birth Defects Research Part A: Clinical and Molecular Teratology. Confined blood chimerism in a monochorionic dizygotic sex discordant twin pregnancy conceived after induced ovulation
In practical terms, this means a girl from an opposite-sex twin pair can carry XY cells in her blood, and her brother can carry XX cells in his. For the vast majority of these individuals, the chimerism is confined to blood and has no effect on sexual development or health. But it can create confusion in genetic testing. Blood-based chromosome analysis might flag an unexpected sex-chromosome result, prompting unnecessary alarm if the clinician is not aware of the twin history.
Why This Does Not Cause Freemartinism in Humans
In cattle, shared placental blood supply between opposite-sex twins has a dramatic consequence. The female calf is exposed to hormones and signaling molecules from her male co-twin through the shared circulation, which masculinizes her reproductive tract and typically renders her infertile. This is called the freemartin syndrome, and it affects the vast majority of female calves born as co-twins with a male.12PubMed. The freemartin syndrome: an update
People sometimes wonder whether something similar happens in humans, and the answer is essentially no. Human placentas very rarely form the kind of extensive vascular connections that cattle placentas do. Most human dizygotic twins have separate placentas entirely, and even those who share one do not develop the large anastomoses that drive freemartinism. The hormonal cross-talk between opposite-sex human twins, if it occurs at all, is far more subtle.
The Twin Testosterone Transfer Debate
Even without freemartin-level effects, researchers have spent decades asking whether females who share a womb with a male twin absorb enough testosterone to be measurably affected. This is the twin testosterone transfer hypothesis, and the evidence is genuinely mixed.
A large Norwegian registry study covering all twin births between 1967 and 1978 found that females with a male co-twin had lower rates of high school and college completion, lower marriage rates, lower fertility, and lower lifetime earnings compared to females with a female co-twin. Because the effects persisted even when the male co-twin died shortly after birth, the authors argued the influence was prenatal rather than social.13Proceedings of the National Academy of Sciences. Evidence that prenatal testosterone transfer from male twins reduces the fertility and socioeconomic success of their female co-twins
However, other researchers have struggled to replicate these patterns. A study of over 300 dizygotic twins born in Aberdeen found no difference in pregnancy or livebirth rates between females from opposite-sex pairs and females from same-sex pairs.14PubMed Central. Testing the twin testosterone transfer hypothesis—intergenerational analysis of 317 dizygotic twins born in Aberdeen, Scotland A broad review of the literature concluded that behavioral evidence for the hypothesis is sparse, though some support exists for subtle effects on perception and cognition, particularly in areas like otoacoustic emissions and spatial ability, and these effects tend to be restricted to females.15PubMed. Evaluating the twin testosterone transfer hypothesis: a review of the empirical evidence
The honest summary is that sharing a womb with a male twin probably does not dramatically alter a female twin’s development, but may produce subtle, hard-to-pin-down differences that vary by outcome and population. The effect, if real, is nothing like freemartinism.
Gender Identity and Opposite-Sex Twins
Biological sex and gender identity are related but distinct, and twin studies have been used to tease apart how much of gender identity is influenced by genetics versus environment. A pooled analysis of 463 twin pairs where at least one twin identified as transgender or gender diverse found that identical twins were concordant for gender diversity about 21% of the time, compared to about 9% for fraternal twins. This substantial gap suggests a meaningful genetic contribution to gender identity.16PubMed Central. Using twin data to examine heritable and intrauterine hormonal influences on transgender and gender diverse identities
Interestingly, the same analysis compared concordance rates between same-sex fraternal twins and opposite-sex fraternal twins. If prenatal hormone exposure from a co-twin drove gender identity, you would expect opposite-sex pairs to show different concordance patterns than same-sex pairs. They did not: concordance was about 9% in same-sex pairs and about 8% in opposite-sex pairs, a difference too small to be meaningful. This finding argues against prenatal sex hormones from a co-twin playing a major role in gender identity development.
A case study of monozygotic male twins discordant for gender identity found neuroanatomical and cognitive differences between them, with the authors noting that unexplained epigenetic effects and later hormone exposure could both play a role.17PubMed. Discordant transsexualism in male monozygotic twins: neuroanatomical and psychological differences The picture that emerges is one where gender identity has strong biological roots but is not determined by any single factor, including the sex of a co-twin.
How Fertility Treatments Affect Twin Sex Combinations
Fertility treatments have increased the rate of twinning overall, but they also shift the odds in some unexpected ways. At a large IVF center, monozygotic twinning occurred in about 2% of pregnancies after fresh day-5 (blastocyst) transfers, compared to about 1.2% after day-3 transfers.18PubMed. Monozygotic twinning: an eight-year experience at a large IVF center This matters because monozygotic twins produced through IVF are subject to the same rare chromosomal accidents as any other monozygotic twins, and a higher monozygotic twinning rate means more opportunities for those unusual outcomes.
A meta-analysis also found that blastocyst transfer is associated with a higher proportion of male births compared to cleavage-stage transfer.19PubMed. The effect of blastocyst transfer on newborn sex ratio and monozygotic twinning rate: an updated systematic review and meta-analysis The reasons are not entirely clear, but male embryos may develop slightly faster in culture, giving them a competitive advantage when embryos are selected for transfer at the blastocyst stage. For couples using IVF, this means the sex ratio among their twins may not follow the same 50-50 expectations that apply to naturally conceived pregnancies.
When Prenatal Testing Gives Conflicting Sex Results
Cell-free DNA testing, widely used during pregnancy to screen for chromosomal conditions, works by analyzing fragments of fetal DNA circulating in the mother’s blood. When a twin pregnancy is involved, particularly one where a twin has been lost early (a “vanishing twin”), the results can become confusing. A study comparing vanishing-twin pregnancies to singleton pregnancies found that vanishing-twin cases had a far higher rate of sex discrepancies on cell-free DNA screening: about 10% compared to just 0.05% in singletons.20Prenatal Diagnosis. Should vanishing twin pregnancies be systematically excluded from cell‐free fetal DNA testing?
The problem is that DNA from the vanishing twin can linger in the mother’s bloodstream for weeks after the pregnancy has become effectively a singleton. If the vanishing twin was a different sex from the surviving twin, the test may detect both XX and XY fragments and return an ambiguous or incorrect sex call. For parents expecting twins early in pregnancy who later learn one has stopped developing, this is worth keeping in mind if subsequent genetic screening gives surprising sex results. The biology of the lost twin, not the surviving one, may be muddying the picture.
Polar Body Twinning
There is one more exotic mechanism worth mentioning. During egg cell maturation, the egg discards extra copies of its chromosomes into small structures called polar bodies, which normally disintegrate. In extremely rare cases, a polar body retains enough cellular machinery to be fertilized by a second sperm. If both the egg and the polar body develop into embryos, the resulting twins share all of their mother’s DNA but could have different fathers’ contributions, somewhat like sesquizygotic twins but through a different route. One confirmed case involved a triploid acardiac twin that developed alongside a normal twin within the same chorion, with genetic analysis revealing that the twins arose from independent fertilization of the egg and its first-division polar body by two different sperm.21PubMed. Genetic studies of an acardiac monster: evidence of polar body twinning in man
Polar body twins could theoretically be different sexes if the two sperm carried different sex chromosomes. Because the polar body rarely survives to produce a viable embryo, confirmed cases are largely limited to abnormal pregnancies. But the mechanism illustrates just how many paths twinning can take, and how many of those paths leave the door open for sex discordance in twins that are more closely related than ordinary siblings.