Fraternal or Identical? Should You Have Your Twins’ Zygosity Tested?


Several years ago I attended a seminar at the University of California Los Angeles (UCLA) where six twin experts discussed their research in the field of twin development and genetics. Before the evening got underway, co-director of UCLA’s Institute for Society and Genetics, Dr. Matthew Norton Wise, related a fascinating anecdote about his adult twins.

“My daughters, for many years, have been fraternal twins,” he said. “All of a sudden, about a week ago, they became identical twins.” It happened, Dr. Norton Wise explained, when he told Dr. Erik Vilain, one of the panelists about to speak that evening, that family and friends continued to confuse his daughters. With the family’s permission, Dr. Vilain then gave the Wise twins a genetic test to accurately determine their zygosity which, as it turned out, was different from what the family had been led to believe.

“So what do you think happens when you go from a fraternal twin for many years to an identical twin?” Wise asked the audience. “The diagnoses I have from my twins is that it’s like ‘coming out of the closet!'” After denying his daughters extremely close relationship for years, he said, many things suddenly seemed natural when they didn’t before. You could say that when it came to understanding his daughters’ bond, Wise had a monumental “ah-ha” moment.

Each of us has a biological as well as a social identity, and no one is aware of that more than twins whose dual identities can sometimes overlap. Genetic testing, therefore, can be a tool to help families better understand the bond that their twins share.

Who Should Have Their Twins Tested?Twin preemies wearing hats and sleeping.

“The most important reason that parents would consider having twins tested is a very much a matter of individual choice, and there are many,” says Robert Jackson, owner of Proactive Genetics, a company that offers DNA testing services. “The most common reason we hear is that parents and or their twins are curious, particularly dichorionic twins [often but not always fraternal] that look very similar. Sometimes the issue of zygosity becomes a matter of debate in the household and among relatives and people will wager on who is shown to be correct.”

For some parents, however, knowing their twins zygosity gives them peace of mind in case one twin is called upon to help the other in a medical emergency since identical twins are compatible donors for each other, says Carlo Lamanna Chapelle of easyDNA, a DNA testing provider. “Organ transplants between identical twins are hardly ever rejected,” he explains. Since identical twins have the exact same DNA, they share the same cell membrane proteins such as the HLA antigen. “It is very difficult to have an organ transplant where the donor and receiver do not have the same HLA antigens,” he explains. “With identical twins this is not an issue as their identical DNA means they have the same HLA antigen.”

Yet for most families, simple curiosity motivates them to have their twins tested. Take Julie Trish, for instance. “My sons have the same eye and hair color and look similar and I was curious if they were identical,” says this Minneapolis, Minn. mom. Since her doctor never confirmed whether her now three-year-old boys were either fraternal or identical, Trish wanted to know for sure especially in regard to future family planning as a mom who gives birth to fraternal twins is four times more likely to have another set of twins in subsequent pregnancies.

The results? “My guys are fraternal,” she says. “It’s nice to know for sure.”

Christine Stewart-Fitzgerald had a similar reason for having her twin daughters tested but had a much different outcome. “At my first prenatal ultrasound, my doctor saw the division between amniotic sacs and said she was 90 percent sure they were fraternal,” says this Carlsbad, Calif. mother to three-and-a-half-year-old twin girls, Alexandra and Julia, and six-month-old singleton Michaela. Yet when her twins were born, Stewart-Fitzgerald could easily tell them apart, convincing her that the diagnoses was correct. “I saw facial differences, so I never thought otherwise.”

But doubts eventually did creep in. Not only did others have trouble telling the girls apart but with each pediatrician visit, their height and weight measurements were nearly identical. Eventually when her daughters reached 18 months old, she ordered a DNA test that she found online and learned the surprising truth—her daughters were in fact monozygotic or identical. Suddenly, the pieces to the puzzle began to fall into place. “Everything seemed to make sense,” Stewart-Fitzgerald says. “While they do have their own personalities, there is a sense of uniformity, togetherness, and being in sync that is very unique.”

Why So Much Confusion?

Although multiple births have exploded during the past two decades, there’s still a lot of misconceptions surrounding their physiology. For instance, nearly 15 percent of all identical twins think they’re fraternal (such as the Wise twins at the beginning of this article). But why? The confusion often starts during the prenatal period when many doctors and clinicians misdiagnose zygosity. Perhaps in their haste, doctors take an educated guess based on one ultrasound examination, never to follow-up again. Sometimes the misdiagnosis happens right after birth when doctors confirm two separate placentas and automatically assume the babies are fraternal. This is by far the biggest fallacy as approximately 20 to 25 percent of identical twins can have separate placentas. On the other hand, two separate placentas from a set of fraternal twins can fuse together giving the appearance of one and the misdiagnoses of identical twins.

In other words, mistakes do happen. Just ask Nancy Forsha. When her twin daughters were born nearly four years ago, her pediatrician told her that upon examination of the girls’ placentas, the hospital believed the girls to be identical. Yet Forsha could easily tell her daughters apart. “We were very skeptical since by then our ‘Baby A’ had brown eyes and a ton of dark hair while ‘Baby B’ had light blue eyes and very blond hair,” explains the Louisville, Ky. mom. “So we finally decided to get an answer for sure by having the zygosity test done.” The result? “They are fraternal in every way, shape and form,” she laughs. “Only half of the markers matched, so they are definitely not identical.”

So when it comes to your twins’ zygosity, the only true way to know is through DNA testing.

How is the DNA Test Performed?

If you order a test online, you’ll receive a kit containing two oral swabs, one for each child. Rub the swab on the inside of each baby’s cheek for a good ten seconds, just long enough for some dead exfoliated cheek cells to adhere to the swab. After the swabs dry, you package them up and send them back to the lab for analysis. You should hear back with the results in 10 to 14 business days. And the cost? Testing usually starts at $100 for twins; $150 for triplets.

Then science steps in. Depending on which company you choose, the lab will analyze between 15 to 16 genetic markers. “Given that identical twins have identical genetic blueprints, to confirm that the two individuals tested are identical we would need a match between all markers,” explains Lammana Chapelle of easyDNA. “The results never fall into a grey area as even just a single, mismatched genetic marker would exclude the two as being identical twins. The result is a definite yes or a definitive no.”

Jackson of Proactive Genetics agrees. “A typical finding is that six to eight out of 15 markers would be different for fraternal twins,” he says. Unless, of course, your twins happen to be half-identical (or semi-identical) where nearly 75 percent of the markers would match up.

But that is a whole different—albeit fascinating—genetic mystery.

A copy of the book Double Duty.

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