Fraternal Twins: Everything You Need to Know

Team Peanut10 months ago6 min read

If you’ve been for an ultrasound and discovered two noses, four arms, and two hearts to steal your one, congratulations! You’re either giving birth to a new kind of superbeing, or you’re about to be the proud mama of a set of twins. But are they fraternal twins, or…?


While you’re scrambling to get yourself together and educate yourself on the ins and outs of paternal and fraternal twins, we thought we’d step in to give you the lowdown.

What are fraternal twins? How do they differ from paternal twins? And, seriously, is there really a third type of twin, too? Let’s jump in.

Fraternal Twins: FAQs

What are fraternal twins?

You’ve heard the word term tossed around a lot in your life—but do fraternal twins mean? Look no further. Here it is, the fraternal twins definition:

Also called dizygotic twins, fraternal twins occur when two eggs are fertilized. So that’s two different eggs. Two different sperm. Two different tiny zygotes. Two different amniotic sacs and placentas.

See? Even as zygotes, they are trying so desperately to differentiate themselves (Spoiler alert: this is the first of many attempts at individuality.)

Fraternal twins are but one type of twin. If you’d believe it, there are (possibly) three kinds. Yes, of course we’re going to tell you what they are. We wouldn’t leave you hanging.

What is the difference between paternal and fraternal?

So, identical vs fraternal—what’s what? As mentioned, fraternal twins are dizygotic, meaning they are the product of two fertilized eggs. Paternal twins, on the other hand, are a little different.

Paternal twins are monozygotic and result from one fertilized egg dividing into two. These are the totally identical type and may even share the same amniotic sac. They are (almost) inseparable.

Learn more ➡️ Different Types of Twins: Monozygotic & Dizygotic

What are the three types of twins?

Hold on. Three types? Paternal twins. Fraternal twins. And…

Disclaimer: the notion of the third type of twin is somewhat contentious. Medical researchers are divided (like fraternal zygotes, if you catch our drift). Some believe that a third type indeed exists and it is a mixture between the fraternal and paternal twin types. This third type occurs when the egg splits as it would with identical twins and only then is separately fertilized.

So the third type then, if it exists, is somewhat of a hybrid form of the two other types.

Can fraternal twins be identical?

Yes and no, depending on your definition of identical. In the sense that they have a strong chance of looking like each other, sure. In the sense that they are formed from the same fertilized egg and therefore have the exact same DNA, complicated.

(It is important to note that this is exactly what the third-twin type theory sets out to explain: how can some fraternal twins look so similar to one another? Does this mean they are identical in a sense? Do their similarities result from the fact that they are the same egg, split into two and then fertilized? So many questions.)

If you have a set of twins that includes a boy and a girl, you’ve got fraternal twins on your hands. That’s because they have a different chromosome combo. (Okay. Retrieve that file from high school biology class. You’ve got this.) A boy? XY. A girl? Signed with kisses: XX.

When it comes to paternal or identical twins, this difference would not be possible as they are the result of a splitting fertilized egg (with the same chromosomes) rather than two separate products of fertilization.

So. Basic differentiator between paternal and fraternal twins? DNA.

Identical twins = identical DNA.

Why fraternal twins?

So that’s all well and good, but it does beg the question, what are the factors that up your chances of being a mama to twins? Turns out, there are quite a few:

1. They look good in your genes. Yes, hereditary definitely plays a role here. If you are a fraternal twin, you may beget fraternal twins. (Luckily, you’ll have some experience with the politics of being a twosome and should be able to parent accordingly.) Chances are also increased if your siblings are fraternal twins or gasp you’ve already had a set of fraternal twins.

2. Your age. If you’re a mama who has amassed a few years of life experience under your belt, it may feel like the bit about your age plays like a stuck record when it comes to pregnancy. It’s because you’re older, they seem to say over and over again about every question asked. And yes, we’re about to push that ol’ chestnut down your throat again. Fraternal twins can indeed be the result of being older. Because there’s more oestrogen as you age. And it may just push an extra egg out of those ovaries at ovulation.

3. If you’ve had fertility treatment. Many fertility drugs serve to stimulate the ovaries. The ovaries may get a little too excited by the whole thing and land up producing more than one egg at a time.

4. You’re already a mama. This is an interesting one—the more babies you have, the higher your chances of conceiving twins. Which is a little cruel of nature, one would think, when space is already at a premium.

One last thing to note as you embark on this journey to becoming the mama of twins, the gestation period is usually shorter—around 38 rather than the more typical 40 weeks. This is because, seriously, there’s only so much your body can handle before it yells, “Everybody out!”

Best buddies forever

Right now, you’re probably experiencing a beautifully-mixed bag of all the feels. While you may be thrilled to hear the pitter-patter of little feet, that’s a whole lot of little feet to find shoes for.

While the thought of an extra mouth to feed may initially seem a little overwhelming, fraternal twins are a real genuine gift. Your babies come with a built-in best friend. They’ll know a bond stronger than any other—after all, they shared a womb before they even shared a room—and that is pretty damn special.

Read also:
Do Identical Twins Have the Same DNA?
How to Conceive Twins