11 Surprising Facts About Fraternal Twins

The majority of twins are fraternal twins (twins that are more like siblings born at the same time than identical twins). Learn more about fraternal twinning, from how they form to how they relate to each other, as well as how they differ from identical twins.


They Develop From Two Separate Eggs

twin facts illustration

Verywell / Katie Kerpel

Fraternal twins each come from their own egg and sperm. The term for this is dizygotic, while identical twins are monozygotic. "Di" means two and "mono" means one. Zygotic refers to the zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby. Monozygotic twins come from a single egg and sperm that splits into two after conception.


They Can Be Different Genders or the Same

Because fraternal twins originate from separate conceptions, they can be boys, girls, or one of each (just like a singleton baby). Chromosomes from the father's sperm determine gender: XX for a girl and XY for a boy. As a result, the chances of fraternal twins resulting in boys, girls, or a combination are the same as for any other baby. Monozygotic—identical—twins, on the other hand, are always the same gender, either two girls or two boys.


They Are As Genetically Similar As Siblings

Just like any siblings, fraternal twins will share about 50% of their DNA. Each person receives half of their DNA from their mother's egg and the other half from their father's sperm, and so any two offspring will have some overlapping qualities. But they aren't the perfect genetic match that identical twins are.


They May Not Look and Act Alike

Monozygotic twins are called "identical" because they often have remarkably similar appearances and characteristics. This stems from the fact that they share the same genetic makeup (or genotype), although their DNA is not necessarily identical.

Fraternal twins, on the other hand, are as alike as any two siblings. They may look very different. They can have different hair color, eye color, stature, and personalities. Or, they may indeed be so similar that they are assumed to be identical, just as some siblings would be remarkably indistinguishable, if only they were the same age.

Twins and multiples are also shaped by their environment after they are born. Some similarities are enhanced because they are raised in the same home, share the same experiences, and are educated in the same schools at the same time.


They Have Two Placentas

During pregnancy, the placenta provides vital sustenance to the baby. In a multiple pregnancy with fraternal twins, a placenta develops for each baby. Sometimes, however, the two placentas fuse together and appear to be one single placenta.

Since some monozygotic twins have one placenta, this can make it difficult to determine zygosity in utero. Because they have their own placentas, fraternal twins are not at risk for some of the conditions that affect monozygotic twins, such as TTTS or monoamniotic twins.


Twinning Can Run in Families

Fraternal twins occur when more than one egg is fertilized. Normally, one egg is released from the ovaries each month, but sometimes there is more than one. Some women release multiple eggs in every cycle, a condition called hyperovulation. Women who hyperovulate are more likely to have fraternal twins.

A tendency toward hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary. A woman who has the gene for hyperovulation can pass it down to her daughter. Then, the daughter's chances of having twins are increased.

Because men carry both X (female) and Y (male) chromosomes, they can also hold the trait for hyperovulation. They can pass it along to their daughters too, increasing their daughters' chances of having fraternal twins.

However, having the gene for hyperovulation does not increase a man's chances of fathering fraternal twins. If a man carries the gene, it doesn't change the ovulation pattern of the mother of his children. She has her own genes governing her ovulation. Instead, it would be his daughter who inherits it through his genes. That is why twins are sometimes assumed to "skip a generation."


Twins Can Be Conceived at Different Times

Usually, one egg is released during ovulation. But in hyperovulation, multiple eggs are released. Sometimes that happens a few days apart. After one egg is fertilized and begins to travel to the uterus for implantation, another egg is fertilized by sperm from a later incident of sexual intercourse. The result is fraternal twins who are conceived a few days apart. This is known as superfetation.

There have even been instances of fraternal twins with different fathers. This occurs when a woman releases multiple eggs and has sexual relations with more than one partner. If an egg is fertilized by sperm from one man, and then another egg is fertilized by sperm from another man, the result is fraternal twins with different fathers. This phenomenon is called superfecundation.


Twinning Rates Vary Across Populations

Population studies have shown that some groups of people have twins much more frequently, while twins are rare among other groups. A 2011 study showed that the highest rates of twinning were found in Central African populations, while Asia and Latin America had the lowest rates of twinning.


Twins Can Be the Result of Fertility Treatments

As medical technology made fertility-enhancing treatments more accessible, the twin birth rate skyrocketed in the late 20th century. Fertility treatments, whether drugs like Clomid or procedures like in-vitro fertilization (IVF), sometimes produce twins or multiples, with the majority of multiple births being dizygotic. Some instances of monozygotic twinning do occur in IVF.


They Can Make a Pregnancy Riskier

Twins are affected by your health and habits during pregnancy. While non-twin siblings would each have a different pregnancy environment, your fraternal twins share increased or decreased health risks due to the pregnancy environment.

Being pregnant with twins puts additional demands on your body compared with a singleton pregnancy. You have a greater risk of pregnancy-induced hypertension, preeclampsia, gestational diabetes, and other conditions. There is also a higher likelihood of premature birth. Both fraternal and identical twins would share this risk.


They Can Result From Many Factors

Many of the factors that influence multiple births only impact fraternal twinning. This is because these factors can encourage hyperovulation, prompting the release of more than one egg per cycle and increasing the chances of having twins.

Heredity, maternal age, how many other children you gave birth to, being taller, and having a higher body mass index are all associated with an increased risk of having fraternal twins. There are weaker associations with using birth control pills, folic acid, and the season of the year. None of these factors have been proven to increase the chances of having identical twins.

A Word From Verywell

Your fraternal twins will share many things throughout their lives after getting a start during the same pregnancy. Enjoy their similarities and differences as they grow. With these facts, you'll be able to tell them about how they are unique.

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