Most people just call them twins. But when you see a brother and sister, or two sisters who look as different as night and day, you realize there's a specific name for it. What is it called when twins are not identical? The scientific term is dizygotic twins, though most of us just say fraternal.
It happens more often than you’d think. Honestly, fraternal twins are the most common type of multiple birth worldwide. They aren't "carbon copies." They’re basically just siblings who happened to share a womb at the same time. Think of it like two separate eggs getting invited to the same party.
The Biology of Dizygotic Twins
Let's get into the weeds of how this actually works. Identical twins come from one egg that splits. Fraternal twins are different. They start when a woman’s ovaries release two eggs during a single cycle—a process called hyperovulation. If two different sperm cells fertilize those two separate eggs, you get fraternal twins.
Because they come from two different eggs and two different sperm, they only share about 50% of their DNA. That’s the same amount of genetic material you share with any of your "singleton" siblings.
Genetics plays a massive role here. You've probably heard that twins "run in families." This is actually only true for fraternal twins. The trait for hyperovulation can be passed down from mother to daughter. If your mom had fraternal twins, your chances of having them go up significantly. Research from institutions like the Mayo Clinic confirms that while identical twinning is a random fluke of nature, fraternal twinning is often tied to family history and even the mother's age.
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Why Fraternal Twins Look So Different
Sometimes they look alike. Sometimes they don't. Since they are dizygotic, they can be different genders. You’ll never see a "natural" boy-girl pair of identical twins because identicals must share the same sex chromosomes. With fraternal twins, it’s a coin toss every time.
I’ve seen fraternal twins where one has curly red hair and the other has straight black hair. It’s wild. But it makes sense when you remember they are just siblings born at the same time. They have different sets of genes for height, eye color, and personality.
Factors That Increase the Odds
It isn't just luck. Several factors can make it more likely for a woman to conceive fraternal twins:
- Maternal Age: Women over 35 produce more follicle-stimulating hormone (FSH). This can cause the body to "accidentally" drop two eggs at once.
- Height and Weight: Interestingly, some studies suggest that taller women or those with a higher BMI have a slightly higher rate of fraternal twinning.
- Fertility Treatments: This is a big one. IVF and medications like Clomid often lead to multiple eggs being fertilized.
- Ethnicity: Rates vary globally. For instance, fraternal twinning is notably higher in certain African populations and lower in parts of Asia.
Superfetation and Superfecundation: The Weird Stuff
Most people think all twins are conceived at the exact same moment. Not always. There’s this rare phenomenon called superfecundation. This happens when a woman releases two eggs and they are fertilized by sperm from two different acts of intercourse.
In some extremely rare (and awkward) cases, this can result in twins with two different fathers. It sounds like a plot from a daytime soap opera, but it’s a documented medical reality.
Then there’s superfetation. This is when a woman is already pregnant, but her body releases another egg weeks later, which then gets fertilized. The "twins" are technically different gestational ages. It’s incredibly rare in humans but it’s another reason why "twins" isn't always a simple label.
The Health Reality of Fraternal Pregnancies
Carrying two babies is a lot of work for the body. Even though fraternal twins have their own separate placentas (dichorionic) and separate amniotic sacs (diamniotic), the pregnancy is still considered "high risk."
Doctors like those at the American College of Obstetricians and Gynecologists (ACOG) monitor these pregnancies closely for things like gestational diabetes and preeclampsia. Because fraternal twins have their own "support systems" in the womb, they generally avoid some of the risks that identical twins face, such as Twin-to-Twin Transfusion Syndrome (TTTS), which is a major relief for parents.
Common Misconceptions
People love to ask, "Which one is the older one?" Even if it's only by two minutes, there’s always a "big" brother or sister. But being the first one out doesn't mean they were conceived first.
Another big myth is that fraternal twins can "turn into" identical twins. Nope. Your DNA is set at the moment of conception. If you started as two eggs, you stay two distinct individuals.
What You Should Do Next
If you're currently pregnant with fraternal twins or just curious about your family history, here are some practical steps to take:
- Check the Placenta Report: If you've already had twins and aren't sure if they are fraternal, look at the pathology report from your delivery. While two placentas usually mean fraternal, it's not a 100% guarantee (some identicals have two if the egg split very early).
- Consider a Zygosity Test: For those who really want to know for sure, a simple DNA swab (Zygosity testing) can confirm whether twins are identical or fraternal with 99.9% accuracy.
- Track Your Family Tree: Look specifically at your maternal side. If your mother or grandmother had fraternal twins, your likelihood of hyperovulation is statistically higher.
- Focus on Nutrition: If you are expecting, fraternal twin pregnancies require more folic acid, iron, and protein than singleton pregnancies. Consult a prenatal nutritionist to tailor a plan for carrying multiples.
Understanding what is it called when twins are not identical is just the start of grasping how complex and cool human reproduction really is. Whether it's genetics, age, or just a roll of the dice, fraternal twins represent a unique biological event where two completely different people share the most intimate beginning possible.