Have you ever met a set of fraternal twins? Many of us have — or, if not personally, we’re at least familiar with a famous set like Mary-Kate and Ashley Olsen. But how much do you really know about this type of twins? There are some pretty darn interesting facts about fraternal twins (also known as non-identical or dizygotic) that you may not even be aware of. And whether you’re expecting, trying to conceive, or just curious about twins in general, these little morsels of information are sure to intrigue.
First, though, let’s start with a quick refresher on what exactly fraternal twins are.
How are fraternal twins formed?
Fraternal twins develop when two separate eggs are fertilized by two separate sperm. Those two fertilized eggs then implant in the uterus and grow. Unlike identical or monozygotic twins, who share 100 percent of their DNA, fraternal twins only share about 50 percent. For this reason, they’re no more genetically similar than non-twin siblings.
What are some surprising facts about fraternal twins?
You know the basics, but prepare to learn some lesser-known details about this type of twin.
1. They are the most common type of twins.
People tend to talk more about identical twins (because how wild, right?), but a twin pregnancy is actually much more likely to result in fraternal twins. Around 75 percent of all twins are fraternal.
2. They have separate supporting structures.
Remember how we said fraternal twins occur when two different sperm fertilize two separate eggs, both of which then implant in the uterus? Because they are two separate eggs when they’re fertilized and implanted, fraternal twins usually develop separate amniotic sacs, placentas, and other supporting structures.
3. They can be the same sex or different.
As is the case with any conception, chromosomes from the father’s sperm determine the gender. Since fraternal twins originate from two separate sperm, each of those eggs could be a boy or girl. That means fraternal twins can both be boys, both be girls, or be a boy and girl. Of course, we are strictly speaking from a genetic standpoint — sex is determined in utero, while gender is a social construct and something a person decides for themselves later.
4. Fraternal twins may have two different fathers.
That’s right — this isn’t just something that you’ve seen in movies. It’s a real phenomenon known as superfecundation, or bi-paternal twins. This happens when a woman releases multiple eggs (known as hyper-ovulation), has sex with more than one partner, and the eggs are fertilized by sperm from different partners.
5. They may not be born on the same day.
Heck, fraternal twins may not even be born in the same week or month! How is this possible? So, let’s say a woman releases an egg during her cycle and it gets fertilized, but her cycle is interrupted. By the time she experiences another cycle, thus releasing another egg, she is already pregnant. Then the second egg gets fertilized, too. This can take place up to 24 days after the first egg’s release, leading to twins born a few weeks apart (although they’re often born in the same general timeframe). This phenomenon is known as superfetation.
6. They might be polar opposites.
Since fraternal twins only share 50 percent of their DNA, they can have very different characteristics and preferences. And while you’ll undoubtedly love their individualism, it can make mastering their sleep routine a bit of a nightmare. This is why identical twins are usually closer than fraternal twins. When it comes to connection, identical twins have a stronger bond than fraternal ones because they share 100 percent of their DNA.
7. They can be hard to distinguish from identical twins.
Here’s the thing: For all of their differences, fraternal twins could still look extremely similar — just as any siblings with shared genetic makeup can look extremely similar. And since environmental factors can lead to identical twins having slight physical differences, many people can’t distinguish fraternal twins from identical. The only 100 percent accurate way to tell the difference is through a DNA zygosity genetic test. This is done by swabbing each twin’s cheek. Besides curiosity, people usually test whether they’re fraternal twins to better understand their health conditions.
8. The highest rate of fraternal twins occurs in Africa.
The rate of this type of twins in Africa is around 14 fraternal twins per 1,000 births. Alternately, the lowest rate of occurrence — six per 1,000 births — is in Asia.
9. Fraternal twins may run in the family.
It’s long been believed by researchers that women who have a history of fraternal twins in their family are more likely to give birth to fraternal twins themselves. In fact, a woman who is born a fraternal twin has a one in 17 chance of conceiving fraternal twins. And a woman who has given birth to one set of fraternal twins is believed to be three times more likely to conceive another set.
10. Fraternal twins are the result of hyperovulation.
We established that fraternal twins are formed when a woman releases more than one egg, and those eggs get fertilized by different sperm. But why did she release more than one egg in the first place? The culprit is hyper ovulation, or the tendency to release multiple eggs during ovulation.
There are myriad possible reasons for hyper ovulation: height (taller women have a higher-than-average rate), high BMI (extra fat stores produce increased estrogen), maternal age (hyperovulation is more likely as you age), multiple births (it’s also more likely if you’ve had multiples), race (highest rates occur in Central African populations), genetics (can be hereditary), and fertility treatments.
11. Many babies are classified as fraternal twins by mistake.
According to a University College London study, 15 percent of parents were told their identical twins were fraternal when they were not.
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