Fraternal Twins: Identical vs. Fraternal Twins

Fraternal twins, or dizygotic twins, happen when two different eggs are fertilized by two sperm. "Di" means two and zygotic refers to the zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby.

The majority of twins are fraternal twins (twins that are more like siblings born at the same time). Monozygotic twins, or identical twins, come from a single egg and sperm that splits into two after conception.

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.

Fraternal twins form in the same way that all humans do, by the union of sperm and egg. Dizygotic twinning occurs when more than one egg is released during ovulation. This is known as hyperovulation.

There are numerous reasons for hyperovulation, including:

There are weaker anecodtal 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 tendency toward hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary. A mother who has the gene for hyperovulation can pass it down to their 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.

If there are fraternal twins on both the mother's and father's sides, your odds for twins goes up even higher.

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 their children. They have their own genes governing their ovulation. Instead, it would be the father's daughter who inherits it through their dad's genes. That is why twins are sometimes assumed to "skip a generation."

As medical technology made fertility-enhancing treatments more accessible, the twin birth rate has skyrocketed. While not all treatments for infertility increase your odds of fraternal twins, many do, including.

Interestingly, while IVF usually results in fraternal twins (due to the transfer of multiple embryos at the same time), some instances of monozygotic twinning can also occur.

Because fraternal twins form from separate eggs and sperm, this can lead to some uncommon circumstances in which the twins have different gestational ages or even different fathers.

Sometimes, hyperovulation 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 person releases multiple eggs and has sexual relations with more than one partner. If an egg is fertilized by sperm from one person, and then another egg is fertilized by sperm from a different person, the result is fraternal twins with different fathers. This phenomenon is called superfecundation.

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. Having a fused placenta increases the chance of low birth weight and intergrowth discrepancy (in which one twin gets more nutrients and oxygen from the placenta and grows bigger than the other).

However, because they typically have their own placentas, fraternal twins are not at risk for some of the conditions that affect monozygotic twins, such as twin-to-twin transfusion syndrome (TTTS) or monoamniotic twins.

Just like other 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, so any two offspring will have some overlapping qualities.

Monozygotic twins share the same genetic makeup, or genotype (although their DNA is not necessarily identical.) This shared DNA is what makes them often have remarkably similar appearances and characteristics.

Unlike monozygotic twins, fraternal twins may look nothing alike, including having different:

Or, they may indeed be so similar that they are assumed to be identical, just as some siblings would be remarkably indistinguishable if 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.

Because fraternal twins originate from separate conceptions, they can both be boys, both girls, or one of each (just like a singleton baby). Chromosomes from the father's sperm determine sex: 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 pair of siblings.

Monozygotic twins, on the other hand, are always the same sex, either two girls or two boys.

Come from their own egg and sperm

Share about 50% of DNA

May look very different

Can be a boy and girl, or the same sex

Come from a single egg and sperm that splits into two

Have same DNA

Look the same

Are always the same sex

All pregnancies carry risks for the pregnant person. Additionally, twins are affected by your health and habits during pregnancy. Fraternal twins share increased or decreased health risks due to sharing the pregnancy environment.

Being pregnant with twins puts additional demands on your body compared with a singleton pregnancy and increases the risk of the following:

While it's not possible to completely eliminate the risks that come with having fraternal twins, you can take some healthy steps to reduce those risks, including:

Your fraternal twins will share many things throughout their lives after starting out in the womb together. Enjoy their similarities and differences as they grow. With these facts, you'll be able to tell them how they truly are unique.