A Woman Gave Birth To Semi-Identical Twins, And Doctors Were Left Totally Perplexed
In 2014, an Australian woman found out that she was having twins. Not by itself news that would make headlines, but when the expectant mother checked in with her doctor, the medical professional was left stunned by what he found. You see, the two fetuses fell neither under the identical nor the fraternal twins category. Instead, they made up an incredibly rare set of semi-identical twins.
Nevertheless, for the Australian woman at the center of this story, her path to motherhood began in typical fashion. Indeed, in February 2019, the BBC reported doctors at the Royal Brisbane and Women’s Hospital as saying that she had become pregnant naturally.
The Routine Ultrasound
As with most moms-to-be, the woman in question underwent the usual pregnancy scans in order to ensure that the fetuses had begun to grow healthily. In most cases, medical professionals employ ultrasound imaging to examine the womb and identify any potential birth defects or similar issues that could be present in a fetus.
Ultimately, then, it was during this routine procedure that Professor Nicholas Fisk noticed something unusual about the Australian mom’s offspring. At first, Fisk said the woman’s pregnancy seemed to be charting a very traditional course. The mother’s ultrasound at six weeks showed a single placenta and positioning of amniotic sacs that indicated she was expecting identical twins, he told the BBC. In time, however, Fisk noticed a single detail on the screen that proved the fetuses couldn’t actually be identical.
Identical vs. Fraternal Twins
Parents of multiples will almost always find that they’re expecting one of two different types of twins: monozygotic or dizygotic. In any pregnancy, you see, sperm fertilizes at least one egg in order to create the zygote cell. Sometimes, though, this cell will split into two embryos, and because the embryos come from the same cell, they have the exact same DNA.
As such, monozygotic twins tend to appear as identical as their DNA and are both the same gender. Despite the similarities in the twins’ appearances, however, they will have different fingerprints. Why? Well, as each baby grows in the womb, they feel around their environment and gather unique markings that differentiate the grooves on their digits. Across the planet, approximately 0.3 percent of births see a set of identical twins brought into the world. What’s more, the likelihood of a woman having lookalike babies is the same everywhere on the planet.
This isn’t the case when it comes to dizygotic or fraternal twins. Fraternal twins are also known as non-identical twins, suggesting, of course, that they do not end up looking the same. That’s largely because the two babies grow from separate eggs fertilized by different sperm cells, and as such, each twin possesses distinct DNA. The pair just happens to share the womb at the same time. So, while fraternal twins can still come out as spitting images of each other—indeed, this can happen with siblings of any kind—their unique DNA means that they can also look totally different, too. And then there’s the possibility of boy-girl dizygotic twins as well, which isn’t the case with identical pairs.
Global Distribution of Fraternal Twins
Instances of fraternal twins don’t have a universally uniform distribution. Instead, different areas of the planet inexplicably have more dizygotic twins than others. Japan has about six sets for every 1,000 deliveries, for example, while areas of India have 15, and in some spots in central Africa, more than 20 births per 1,000 bring two bouncing, but not generally identical, babies into the world.
It’s worth noting, too, that fertility treatments and medications have boosted the number of fraternal twins. Typically, these drugs cause mothers to hyperovulate, which means their bodies emit numerous eggs at once. It’s the perfect scenario, then, for more than one embryo to form. Because of this phenomenon, the numbers of multiple births—from twins to triplets to quadruplets and beyond—have spiked in the U.S.
The Anomaly in the Ultrasound
With all of this information in mind, it becomes easier to understand why Fisk found himself confused when he performed an ultrasound for the Australian mom-to-be in 2014. At first, the doctor saw a pair of amniotic sacs that were each on the same placenta—usually a surefire sign that the mom would have identical twins. After that, Fisk told the mom that she’d have to come back for another checkup to determine the sex of her fetuses, although it may have appeared safe to assume that they’d either be two boys or two girls.
But at the mom’s 14-week ultrasound, Fisk found himself facing the unexpected. Yes, the mom had one boy and one girl on the way, even though the fetuses were growing in a monozygotic manner. According to a March 2019 report by The New York Times, Fisk thought to himself at that moment, “It doesn’t add up.”
A Rare Discovery: Sesquizygotic Twins
Eventually, though, he realized that he had overlooked a third twin category—one so rare that there’s only one other recorded instance. Specifically, Fisk discovered that the twins were semi-identical, also known as sesquizygotic. Therefore, the fetuses had formed after one egg in their mother’s womb had been fertilized by separate sperm.
As a consequence, while the twins each shared differing amounts of their father’s DNA, they both had 100 percent of their mom’s. “Broadly, they are about three-quarters identical,” Fisk told The New York Times, before adding that such a situation is “almost as rare as hen’s teeth.”
How Semi-Identical Twins Form
In theory, you see, such a conception shouldn’t be feasible, especially considering the two ways in which twins typically form. In both scenarios, one sperm plus one egg equals one baby. “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.” Owing to this excess of genetic material, Fisk added, the resulting fetus ultimately wouldn’t survive.
But semi-identical twins nonetheless occur because they each somehow receive only the amount of genetic information they need. “Some of the cells contain the chromosomes from the first sperm, while the remaining cells contain chromosomes from the second sperm,” Fisk’s colleague, Dr. Michael Gabbett, explained to CNN in March 2019.
Genetic Analysis
As such, Gabbett said this resulted in the twins sharing only a proportion, rather than 100 percent, of the same paternal DNA. In fact, staff at the Royal Brisbane and Women’s Hospital embarked on a painstaking examination of each twin’s DNA, comparing all of the female fetus’s chromosomes to her brother’s. In some stretches of the twins’ DNA, they shared the exact same genetic information; in other parts, though, they were different.
Ultimately, this amounted to the fetuses sharing approximately 89 percent of their DNA. The process also made Fisk the first doctor to identify semi-identical twins during their mother’s pregnancy. The only other recorded case of semi-identical twins occurred in 2007. In that situation, however, the female twin required medical care because she had what is termed ambiguous genitalia. When doctors examined the female fetus’s genetic code, they realized that she and her twin had identical DNA from their mother, but only about 50 percent of the same code from their father.
Thriving and Expanding the Research
Neither of the Australian twins had to face this hurdle, though. Indeed, they came into the world healthy, and as Fisk told The New York Times, they continue to thrive. “At age four, they’re doing well and meeting their developmental goals,” he said.
Still, once Fisk and researchers from two Australian universities realized that they had this rare set of semi-identical twins on their hands, they couldn’t help but wonder how many other sesquizygotic babies existed out there. Sure, there was the 2007 case, but they thought that other twins may have been misidentified or perhaps that their very DNA may not have been chronicled.
To prove the rarity of the semi-identical twins, Fisk and his colleagues from the University of New South Wales and the Queensland University of Technology went on to examine the DNA of close to 1,000 sets of fraternal twins.
Confirming the Findings
Fisk went on to describe the monumental test to CNN, saying, “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.”
With the investigation finally complete, the team could be sure of their initial notion: the semi-identical twins delivered in Australia were indeed extremely rare. Nevertheless, before the researchers shared their findings through the New England Journal of Medicine, they had to be thorough. Fisk later acknowledged to CNN, “We needed to convince rightly skeptical reviewers that this data was really robust.”
As it turned out, the group’s work piqued the interest of other sets of twins. Fisk told The New York Times that he had received numerous emails from those who thought that they and their twin may have gone through life without knowing they were more than just a fraternal pair.
Lingering Questions
Even with Fisk and his team’s finding, many questions remain about semi-identical twins. Dr. Mindy Christianson of the Johns Hopkins School of Medicine said that the research may have only scratched the surface of what can happen when twins are conceived. “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.”
Nevertheless, Fisk and his colleagues did consider this possibility, and yet their examinations of almost 1,000 fraternal twins didn’t uncover any semi-identical pairs among the study group. For Fisk, though, the loose ends that remain are all part of the fun of being a researcher of twins, even if he acknowledges that there is still work to be done on the subject. For instance, although identical twins seem to be a clear-cut phenomenon, Fisk believes that there could be more to be learned understanding how they form.
Outro
Yet while the semi-identical twins at the center of this breakthrough had practically been proved to be medical miracles, their family seemingly don’t want the pair to become notorious. In any case, the twins’ identities have not been released to the media, although there has since been an update on how they’re thriving. In February 2019, Gabbett told ABC, “The four-year-olds look very similar, they’re very cute, and they’re doing very well.”
