What are Fraternal Twins?

Diane Goettel
Diane Goettel

Fraternal twins are siblings who are conceived at the same time of two separate fertilized eggs. They are also sometimes called "biovular twins" or "non-identical twins." In fact, it is quite common for fraternal twins to be defined in comparison with identical twins. Identical twins are two siblings who are born of a single fertilized egg that splits in order to create two fetuses instead. One of the key features of identical twins is that they look incredibly similar, or identical. Also, identical twins always share the same gender.

Fraternal twins are sometimes one boy and one girl.
Fraternal twins are sometimes one boy and one girl.

Unlike identical twins, fraternal twins can look quite dissimilar as siblings sometimes do. In fact, it is common for fraternals to be confused for siblings that simply happen to be similar in age. This is especially true in late childhood, adolescence, and later in life. Furthermore, while these twins can be of the same gender, it is also possible for a pair of this kind of twins to be made up of one boy and one girl. This is because, instead of sharing all of their genes as with identical twins, fraternal twins share an average of 50 percent of their genes.

Fraternal twins develop from two zygotes.
Fraternal twins develop from two zygotes.

Another term for twins of this type is "dizygotic twins" which is sometimes referred to in a shortened term as "dz twins." This simply means that the twins are the result of two zygotes instead of one, as in the case with identical twins. Zygote is the scientific term of a fertilized egg. In the instance of identical twins, or "monozygotic twins," only one zygote is present at the moment of conception.

Fraternal twins can look quite dissimilar, as siblings sometimes do.
Fraternal twins can look quite dissimilar, as siblings sometimes do.

Much later in life, it is possible for identical twins to be confused with fraternal twins. This is because general health, one's level of personal care, and one's lifestyle can alter the manner in which one ages. If the two members of a pair of identical twins lead very different lives, it is possible for them to look more and more dissimilar as they age. Cosmetic choices such as hair style, clothing, and makeup can also make identical twins look more like they're fraternal.

Boy/girl twins are always fraternal -- or dizygotic, as they can only form from separate eggs fertilized by separate sperm.
Boy/girl twins are always fraternal -- or dizygotic, as they can only form from separate eggs fertilized by separate sperm.

There are some groups that are more likely to produce fraternal twins than others. Dizygotic twins are more common among older mothers, especially women who conceive after the age of 35. They are also quite common in Japan and in some parts of Africa. There is some evidence that there is dz twins are often the result of a genetic predisposition.

Twins may have rivalries and strained relationships just like siblings who are several years apart.
Twins may have rivalries and strained relationships just like siblings who are several years apart.
Fraternal twins are more common in women who conceive after age 35.
Fraternal twins are more common in women who conceive after age 35.
Diane Goettel
Diane Goettel

In addition to her work as a freelance writer for wiseGEEK, Diane is the executive editor of Black Lawrence Press, an independent publishing company based in upstate New York. She has also edited several anthologies, the e-newsletter Sapling, and The Adirondack Review. Diane has a B.A. from Sarah Lawrence College and an M.A. from Brooklyn College.

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Discussion Comments


@wavy58 – Sometimes fraternal friends can favor a whole lot. My sister and I always get asked if we are twins, and we always tell people, “Not in the way that you think.”

We share a birthday, but we didn't share an egg. Our noses are similar, but our lips are different, and our eye color is also different.

Sure, from a distance, it might be easy to mistake us for identical twins, but once you take a close look at our faces, it shouldn't be hard to tell us apart. Her eyes are more green, but mine have a bluish tint, and my lips are slightly fuller than hers.

We always think it's funny when relatives who we only see once a year get us confused. We keep the same hairstyles on purpose, just to see who really knows us!


@burcinc – Fraternal twins with different fathers can look so dissimilar that no one would know they are related. I had some close friends in school who were fraternal twins, and since their mother was a prostitute, it was no wonder that they had different fathers.

She didn't seek to find who the fathers were, but it was obvious that there was more than one. Since one child was half black and the other half Asian, no one believed they were even related. Everyone thought they were both adopted.

This is a bizarre situation, and I realize that with most women, this would not happen. It really made fitting in and having a sense of family hard for my friends. I don't think they ever sought to find their fathers, because knowing how they were conceived didn't motivate them.


I think that fraternal twins have less of a chance of hating each other than identical twins. I knew a set of identical twins in high school that could not stand each other.

They had grown tired of people confusing one for the other. Also, when one twin decided to have the same hairstyle as the other, the first one would get mad, because she felt that her identity was being robbed, since no one could tell them apart without distinguishing factors like hair and clothing.

I met a set of fraternal twins in college, and they were as close as best friends. They never had identity issues, because you could easily tell who was who. I know that you always hear of identical twins sharing a strong bond, but these fraternal twins were much closer than the identical twins I knew.


My best friend had never wanted children, but she wound up pregnant with twins. At first, she was devastated. She could not imagine having one child, yet alone two!

After awhile, though, she got excited about it. Her doctor told her that she would be having fraternal twins, one a boy and one a girl.

Since her husband had always wanted a son, he was stoked. His joy was contagious, so she really got into the idea.

Unfortunately, she lost the son before he was ever born. This crushed my friend, and to make matters worse, she couldn't grieve too deeply, or this could place strain on the unborn girl. Once she gave birth to her daughter, her sorrow turned to joy.


@alisha-- Yea, same gender fraternal twins are pretty common. Did you know that the Olsen twins are fraternal twins? Everyone thinks that they're identical twins because they look so similar but they're not.

There is something else that can happen with fraternal twins that is very rare and pretty weird too. The twins can have different dads! I saw this on a TV program where this woman had fraternal twins and DNA tests showed the fathers to be different.

This is such a rare thing to happen because the woman would have to have sex with two men within the same period of time and a sperm from each men would fertilize two eggs that the women ovulated. Then the eggs would have to attach to be fraternal twins! It's so odd but it actually happened!


@burcinc - I always thought having twins would be great too - although I wanted to have two girls. Even though it would be hard to take care of two infants at once, it would be nice to go through all those stages at one time.

I think once they were older it would be a lot of fun. They would be great friends and companions for each other. Many moms of young twins look pretty tired and worn out and I can see how that would be possible.

If they weren't on the same schedule, you could get worn down pretty fast.

If someone finds out they are pregnant with twins, can they tell if they are having identical or fraternal twins in the womb? Will they be able to know before they are born if they will look alike or not?


@burcin-- I have fraternal twins too and one is a boy and the other is a girl! I've actually not seen any same-gender fraternal twins until now. I thought that maybe they're always different genders. That's interesting that they look alike too.

My twins look very different. Obviously, there is a gender difference but some siblings end up looking very similar too and my fraternal twins are so different appearance wise. One looks like me with brown hair and brown eyes, the other one has lighter hair and blue eyes like his dad.

And like the article said, all the kids at school thought that they're siblings. They only realized it when the teacher mentioned their birthday in class.


Since fraternal twins' DNA is only 50% the same, it is easy to see how some fraternal twins who are the same gender don't look like they are twins.

I have a friend who has a boy who is 5 and fraternal twin boys who are 4. She really has her hands full! All of the boys look similar especially since they are so close in age.

The 5 year old boy looks more like one of the twins than the other twin does. Many times people will ask her if those 2 are twins, or if she even has triplets.

You can see a close bond between the twins though. Right now it is hard to tell if this is because they are the same age or because they are twins - maybe both.

As they get older it will be interesting to see if this bond strengthens or if they become more independent of each other.


I think fraternal twins can look very similar in appearance too. My sister has fraternal twins, they're both boys and they look like identical twins.

People always think that they are identical twins until they ask, they look that much alike! I wonder if they will look very different when they're much older. They're even the same in terms of height and weight. I guess we'll see with time what happens.

I had never heard about the genetic predisposition for fraternal twins though. As far as I know, my sis is the only one who had twins in our family. I would love to have fraternal twins too though. It would be great if one was a boy and the other a girl! It'd be like two pregnancies in one!


@SarahSon - I don't know if this a common difference or not, but I have nieces who are identical twins and it is amazing how strong that bond is between them.

Not only do they look alike, but they also think and act alike - sometimes it is almost scary. I always thought it would be neat to have this close of a bond with someone.

When my sister found out she was having twins, she was really excited. She didn't know what her chances were of having fraternal twins or identical ones, because there weren't any twins on either side of their families.

She was just glad they were born healthy and they have brought a lot of joy and laughter to their lives.


It seems like most of the sets of twins I know are identical twins, but maybe because it is so easy to identify them.

There might be more fraternal twins than what I realize because they don't look like each other.

I know one set of fraternal twins and they are both boys. If you didn't know they were twins, you would never be able to guess it.

One of them has blonde hair and one of them has dark brown hair. They are both tall, but they don't have any of the same features or mannerisms.

Even as they have gotten older, I don't see many similarities between them. They also don't seem to have as close of a bond as a lot of the identical twins I know. I wonder if this a common difference between identical and fraternal twins?

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