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Jayme Burrows/Stocksy United

If you’re a parent of twins, one of the first questions you’ll probably get asked is if your children are identical or fraternal twins. If your twins are identical, the excitement is often palpable once people find out.

The idea of dressing children in the same clothes and struggling to tell them apart is such a fun concept in pop culture. Yet after centuries of societal assumptions that identical twins were well, identical — right down to genetic composition in recent times — the reality might be somewhat different than we thought.

Unless you’re well versed in biology and conception, many people are confused about the distinguishing difference between fraternal and identical twins. It’s about more than just two people looking like each other — or not.

Fraternal twins (dizygotic) are conceived when two eggs are fertilized in the same ovulation window. Identical twins (monozygotic), on the other hand, are conceived from a single egg that splits into two distinctive embryos.

To break it down even further, identical twins are created from the combination of a single egg and sperm. In contrast, fraternal twins are the product of two separate eggs being fertilized by different sperm.

So, whereas identical twins come from the same genetic material, fraternal twins do not. (Fraternal twins will have genes in common the way any siblings from the same biological parents do.)

Of the two twin scenarios, identical twins are more likely to occur by chance. Although giving birth to fraternal twins can also happen naturally, this type of twin birth is more commonly seen in people who undergo fertility treatments.

That’s because fertility drugs can increase the number of eggs released in a cycle — or, with in vitro fertilization, multiple embryos can be reinserted into the uterus.

According to a 2021 study, an estimated 15 percent of identical twin pairs may have one member that exhibits significant genetic variation from their twin.

In the lab study, researchers tried to figure out how many genetic mutations that cause monozygotic twins to have different DNA typically occur. In the 381 twin pairs analyzed, they found a median of 14 post-zygotic mutations differing between a pair of twins. Yet there was variation: 39 twin pairs differed by more than 100 mutations, while 38 pairs didn’t differ at al.

Though the average number of genetic mutations leading to differences in DNA between identical twins may be small, it’s still a major revelation given the common assumption that identical twins are genetically indistinguishable.

Even though identical twins do share a significant amount of similar DNA, there are clear genetic mutations between each member, proving that they’re not carbon copies of each other.

So, why do some twin pairs have more genetic variation than others?

There’s a link between when twinning occurs and the increased potential for genetic mutations. Earlier twinning splits mean that each zygote has more time to form DNA independently as it continues to grow, leading to a higher chance of mutations.

A common question is when does the embryonic split — or twinning — actually happen in identical twins.

It turns out that there are a few stages where an embryo can split into two distinctive zygotes. This can happen anywhere from day 3 through day 13 following conception.

3-day separation

As the term “identical twins” implies, some are created when an embryo transitions from simply being a single fertilized cell into a zygote.

In a singleton (one baby) pregnancy, the zygote splits into two cells — usually around day 3 — but the cells remain connected to each other. But in twinning, rather than staying connected, those two cells fully separate into two distinct entities.

Twins formed during a two-cell split are more likely to have the highest genetic mutations — or diversity — between them. The earlier the split is, the more mutations can occur.

5-day separation

In some scenarios, the zygote continues to multiply well beyond the two-cell stage. And it may take until day 5 for twinning to occur. These types of twins are known as mirror image twins that each have a separate placental sac.

Understandably, twins from this type of split will have lower variation between their DNA than those from a two-cell split.

Note that most twins are produced between days 3 and 7 post-conception.

9-day separation and later

The latest phases for twinning can occur around day 9 and beyond. In a 9-day split, these twins are still considered mirror image babies. Compared with 3- and 5-day twin splits, this group is most likely to have the least genetic mutations.

But the risk increases that they may share a placental sac, which can be dangerous. The most common concern is that the umbilical cords can get tangled, leading to complications.

And splits that occur after day 10 have a higher risk of resulting in conjoined twins.

While most identical twins do share almost completely identical DNA, some do not.

Again, because the embryos develop independently after the zygotes split, identical twins can have different health conditions, physical differences, and other small changes that make each member of the pair distinguishable from the other.

Some genes or traits may get suppressed during pregnancy, which can result in one twin being slightly taller or thinner or having a hair color that’s not quite like the other. Or one twin might receive more nutrition in the womb than the other, resulting in a slightly different physical appearance than their sibling.

No matter how much your two bundles of joy might look identical, at a genetic level, they’re not quite the same! But you can still have fun dressing them up in the same outfits — until they’re old enough to protest, that is.

Know that while your twins might look physically indistinguishable, small genetic mutations that are usually harmless make them each genetically unique.

And in some cases, these mutations may mean that one child is taller, has slightly different features, or might even experience different health concerns than their twin.