A Woman Gave Birth To Semi-Identical Twins, And Doctors Were Left Totally Perplexed

In 2014, an Australian woman discovered that she was carrying twins. On its own, that hardly seems like headline news. But when the expectant mother went in for a routine check, her doctor was left utterly astonished by what appeared on the monitors. The two fetuses fit neither the identical nor the fraternal twin profile.

Instead, they formed an exceptionally rare pair of semi-identical twins.

Even so, the mother’s journey began quite ordinarily. In February 2019, the BBC cited clinicians at the Royal Brisbane and Women’s Hospital confirming that she had conceived naturally. Like most expectant mothers, she scheduled standard ultrasound screenings to make sure the babies were developing well.

Obstetricians rely on ultrasound scans to check the uterus and catch any developmental anomalies early on. It was during one of these routine appointments that Professor Nicholas Fisk caught sight of something genuinely baffling.

At the start, Fisk noted that the pregnancy looked entirely conventional.

“The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins,” Fisk told the BBC.

As time went on, though, a single detail on the screen shattered that assumption.

Parents expecting multiples almost always fall into one of two groups: monozygotic or dizygotic. In a typical pregnancy, a single sperm fertilizes an egg to create a zygote. On occasion, this single cell divides into two separate embryos. Because they originate from the exact same fertilized egg, they share 100% of their DNA.

These monozygotic—or identical—twins share the same genetic blueprint and are consistently the same sex. Yet despite matching physical traits, their fingerprints remain distinct. As fetuses move around inside the womb, slight contact variations shape the faint ridges on their fingertips into wholly individual patterns.

Globally, identical twins account for roughly 3 in every 1,000 births. This rate remains virtually constant across populations worldwide.

Fraternal, or dizygotic, twins tell a very different story. Known as non-identical twins, they develop from two separate eggs fertilized by two different sperm cells. Genetically, they are no more alike than ordinary siblings born years apart; they just happen to share the womb at the same time. While some fraternal twins bear a striking resemblance, others look completely different.

Fraternal twins can also be boy-girl pairs—something biologically impossible for typical identical twins. Furthermore, fraternal birth rates vary widely across regions. In Japan, fraternal twins occur in roughly 6 out of every 1,000 deliveries, whereas parts of India see around 15 per 1,000. In certain regions of Central Africa, that figure climbs past 20.

Fertility treatments have noticeably nudged these numbers upward as well. Ovulation-inducing medications frequently prompt ovaries to release multiple eggs during a single cycle, boosting the likelihood of multiple simultaneous conceptions and driving up twin and triplet births.

Knowing this background helps explain Fisk’s utter confusion during that 2014 scan. His initial view showed two amniotic sacs sharing a single placenta—nearly always the hallmark of identical twins.

At that stage, Fisk advised the mother to return later to confirm the babies’ sex, assuming they would turn out to be either two boys or two girls.

At her 14-week ultrasound, however, everything flipped. The scan clearly revealed one boy and one girl, even though the structural development pointed squarely to monozygotic twins.

According to a March 2019 piece by The New York Times, Fisk recalled his immediate reaction:

“It doesn’t add up.”

Gradually, he realized he was looking at an elusive third classification—one so unusual that only a single other case had ever been documented. The twins were sesquizygotic, or semi-identical.

Biologically, a single maternal egg had been fertilized simultaneously by two distinct sperm cells. As a result, both children shared 100% of their mother’s DNA, but inherited differing proportions of their father’s genetic material.

“Broadly, they are about three-quarters identical,” Fisk told The New York Times, adding that such an occurrence is extraordinarily rare.

In textbook biology, this kind of conception should not produce a viable pregnancy. Usually, one sperm fertilizes one egg.

“When a sperm enters an egg, the membrane locks down to stop any other sperm from getting in,” Fisk explained.

“Even if another sperm got into an egg, you would end up with three sets of chromosomes. And owing to this excess of genetic material, the resulting fetus ultimately wouldn’t survive.”

Yet these twins survived because their developing cells managed to parcel out the genetic material properly.

“Some of the cells contain the chromosomes from the first sperm, while the remaining cells contain chromosomes from the second sperm,” Dr. Michael Gabbett, Fisk’s colleague, told CNN in March 2019.

“This resulted in the twins sharing only a proportion, rather than 100%, of the same paternal DNA.”

Specialists at the Royal Brisbane and Women’s Hospital undertook an exhaustive genetic mapping of both infants, aligning the girl’s chromosomes against her brother’s. Along certain stretches, their sequences matched completely; along others, they diverged. All told, the twins shared around 89% of their total DNA.

This breakthrough marked the first time sesquizygotic twins had ever been identified during gestation.

The only previously recorded instance occurred in 2007. In that case, clinicians discovered the condition only after the female twin presented with ambiguous genitalia, prompting deeper genetic testing that revealed maternal identity alongside a 50% paternal match.

The Australian twins faced no such complications. They were delivered healthy, and their development remained completely on track.

“At age four, they’re doing well and meeting their developmental goals,” Fisk confirmed to The New York Times.

The discovery led Fisk and his research partners across two Australian institutions to wonder whether other semi-identical births had simply slipped under the radar, misclassified as ordinary fraternal pairs.

To determine just how rare this was, Fisk joined researchers from the University of New South Wales and Queensland University of Technology to evaluate genetic samples from nearly 1,000 sets of fraternal twins.

“We needed to confirm the results with really exhaustive studies in multiple labs in multiple states and countries, and with tests involving millions of genetic variants,” Fisk explained to CNN.

“We needed to convince rightly skeptical reviewers that this data was really robust.”

The validation confirmed their suspicions: true semi-identical twins represent an astonishingly rare anomaly.

When the findings were published in The New England Journal of Medicine, the story drew widespread public attention. Fisk noted that his inbox filled with inquiries from fraternal twins wondering whether they might secretly belong to this unique third category.

The case also opened new questions for reproductive science. Dr. Mindy Christianson of the Johns Hopkins University School of Medicine pointed out that current research might only be scratching the surface.

“The recent case was detected because the twins were male and female,” Christianson told The New York Times.

“There may be other same-sex twins out there that are also semi-identical.”

Even though screening nearly 1,000 fraternal twins yielded no additional sesquizygotic pairs, researchers like Fisk view the open questions as fertile ground for future embryological discovery. Even longstanding assumptions regarding identical twinning continue to reveal unexpected biological nuances.

Despite living through a documented medical wonder, the family chose to preserve their privacy. Their names have never been made public, though occasional clinical check-ins confirm that both children remain healthy and energetic.

“The four-year-olds look very similar,” Gabbett remarked to ABC.

“They’re very cute, and they’re doing very well.”