What Happens if Twins Have a Baby Together? Various Scenarios

There exists a diverse range of twins, consisting of a total of seven distinct types, each of which is fascinating mainly due to their uncommonness.

Having questions about twins is completely natural, and many people wonder what would result if twins reproduced with another set of twins.

What happens if twins have a baby together? If two sets of identical twins reproduced, the resulting children, known as quaternary twins, would be genetically more like siblings than cousins and would likely look very much alike. If two sets of fraternal twins reproduced, the children would be ordinary cousins.

While most people know about identical and fraternal twins, one kind that is very rare is quaternary twins.

Quaternary twins occur when one set of identical twins have children with another set of identical twins. 

This is a very rare phenomenon, and there are currently only about 300 pairs of quaternary twins in the world. Twins have a tendency to create a lot of curiosity in people, and these quaternary twins certainly seem to be very unique. 

Quaternary Marriage 

Quaternary marriage is when two sets of identical twins marry each other. For example, say two identical twin brothers marry a set of female identical twins.

One of the most famous cases of quaternary marriage in modern times is the Saylor family in Virginia in which identical brothers married identical sisters. 

Identical Twins Marrying Twins

Identical twins share the exact same genetic code as they are the result of one egg splitting in half during pregnancy.

Therefore, when identical twins marry another set of identical twins, instead of four different genetic codes, essentially, there are only two

What Are Quaternary Twins?

Quaternary twins are the children produced when identical twins conceive with another set of identical twins.

The children from these relationships are legally cousins but in a genetic sense, they are siblings. Their genetic codes are very similar due to the fact that their parents have the same exact DNA as their aunt and uncle. 

Another term for quaternary twins is twin cousins. Due to their genetic similarities, these twin cousins have a much higher likelihood of looking quite similar to each other.

To have twin cousins, the children don’t have to be born close together. They are “twins” simply because their parents have the exact same DNA. Therefore, quaternary twins could be born 12 years apart and still considered twin cousins. 

Twins Marrying Twins and Having Twins

Identical twins who marry identical twins actually don’t have a higher probability of having identical twins themselves.

There is some research that male identical twins have more sets of identical twins, but that is not always because they marry an identical twin.

Identical twins do not run in families and have no real known cause. Fraternal twins are much more likely to occur due to family history.

Couples in quaternary marriage will most likely have children who look very similar due to their parents being genetically identical. Fraternal twins do not have to look alike as they are two siblings with their own code who happened to share a womb. 

Children of Fraternal Twins

Children of fraternal twins are legally and genetically cousins. Fraternal twins share 50% of their genetic coding. They are no more similar than regular siblings, just with the uniqueness of having been in utero together. 

What Happens if Two Sets of Fraternal Twins Have Babies?

The babies that result from these types of relationships would be the same as if sisters married brothers.

While interesting from a cultural and social perspective, genetically, these children would not be very unique.

Again, fraternal twins are the same as siblings and share 50% of their DNA, so if fraternal twins married another set of fraternal twins, their kids would have four different sets of genetic codes. 

What if Fraternal Twin Siblings Had a Baby?

This would be considered the same as a brother and sister having a baby as fraternal twins are regular siblings. This is also considered incest. Incest can cause a whole host of birth defects and higher infant mortality. 

Children of Identical Twins

The children of identical twins share a higher percentage of shared DNA than other cousins.

What Happens if Two Identical Twins Have a Baby? 

Reproduction has come a long way in recent times. However, with identical twins, the twins are either both female or both male, so the chances of them having a child together are nonexistent.

Reproduction still requires the presence of both male and female genetic material. 

When Identical Twins Have Babies, Are They Half Siblings?

Because their parents are identical genetically, children of identical twins are more like half-siblings.

My own life exhibits this phenomenon fairly well. My mother is an identical twin, and my aunt’s children are genetically like half-siblings to my sister and me, whereas, my fraternal twins’ future children will be cousins in the literal sense of the word.

Fraternal twins share 50% of their DNA, whereas identical twins share 100% of their DNA. 

Related Questions: 

Can Cousins Be Twins?

The only way for cousins to be twins would be in the case of quaternary marriage. Quaternary twins are when two sets of identical twins reproduce with another set of identical twins.

Their children would have very similar DNA and have a high chance of looking very similar.

What Are the Chances of Having Two Sets of Fraternal Twins? 

This question strikes fear in the hearts of parents of twins who are looking to expand their family again. The odds of having a second set of fraternal twins after already having had fraternal twins is 1 in 12!

Those odds are affected by many things that can make twins more common than once in a lifetime. Older maternal age and infertility treatments are two of the explanations for a second set of fraternal twins born to the same family.

However, the chance of having two sets of identical twins born to you is about the same as your chance of getting struck by lightning. 


Twins are fascinating to many people simply because they are rare. Telling someone that you have twins often results in awe from other parents and comments, such as “I can barely handle one. How did you do two?”

If fraternal twins spark interest, quaternary twins born to two sets of identical twins would cause even the most somber of people to stop and stare.

However they are made and whatever their genetic code, having twins is a fun and exhausting experience.