Differences Between Identical and Fraternal Twins

Due to the increased use of assisted reproductive technology, the incidence of identical and fraternal twins has increased. Do you know the differences between them? Discover them here!
Differences Between Identical and Fraternal Twins

Last update: 16 June, 2019

Identical and fraternal twins share many common characteristics and differences. Although identical twins are more common, how do you distinguish them from fraternal twins? You can distinguish them with the method of conception, genetic code, blood type, and appearance.

Differences between identical and fraternal twins

Twins aren’t always identical. In fact, they can differ in many ways. The two kinds of twins can be distinguished mainly by their development.

Identical twins are monozygotic, meaning they develop in a single zygote that divides into two embryos. This happens nine days after fertilization. On the other hand, fraternal twins are dizygotic, meaning they develop in different zygotes and each is fertilized by a different sperm.

Their genetic codes are also different. Fraternal twins, as common siblings, aren’t identical. However, identical twins are very similar.

While fraternal twins develop in separate amniotic sacs, identical twins develop in the same amniotic sac. This means that twins can often be identified in the womb.

Identical twins can be very similar in almost every way. However, due to environmental factors, they can sometimes change their appearance and only look somewhat alike. Meanwhile, fraternal twins, just like other siblings, are different.

Characteristics of twins

While identical twins come from the division of a single cell, fraternal twins come from different cells. Here are their characteristics:

Identical twins

Differences Between Identical and Fraternal Twins

These siblings don’t have an exactly similar genome. In fact, both twins’ blood cells may have differences in their DNA sequence.

If they receive inconsistent nutrition during gestation, both twins’ appearance may change during growth. Although some identical twins can be of different gender, most of them are of the same gender.

Fraternal twins

Dizygotic twins stem from the fertilization of two different eggs by two different sperm. Fraternal twins’ genomes are different, as they’re siblings who were conceived at different times. Also, they can be of the same gender or of different genders.


The odds of conceiving identical twins is the same worldwide: they occur in three out of every thousand births. However, the odds of having fraternal twins varies due to several factors, including the home country.

For example, in Japan, six of every thousand births can be fraternal, while in India the odds are 15 in every thousand births.

In addition, now it’s more common for mothers over 35 years old to conceive fraternal twins. This possibility is boosted in pregnancies through in vitro fertilization (IVF).

It’s important to note that fraternal twins can be conceived when a woman releases more than one egg during her menstrual cycle. Therefore, if you’ve already had fraternal twins, you’re four times more likely to conceive them again.

Some drugs such as clomifene may increase the likelihood of having fraternal twins by 10% since they cause superfecundation.

“While identical twins come from the division of a single cell, fraternal twins come from different cells.”

Risks of twin pregnancies

Is it risky to have identical and fraternal twins? Yes. However, the gestation period is successful in most cases. The survival rate of twins is 60%. However, there’s a risk that the umbilical cord will tangle around the babies, which can restrict oxygen flow and cause brain damage.

On the other hand, they’re at risk of low birth weight and preterm delivery. In addition, the gestation period is usually 38 weeks instead of 40 weeks. Therefore, you should go see your doctor for regular checkups and follow their instructions carefully.

Differences Between Identical and Fraternal Twins

Is conceiving identical or fraternal twins genetic?

There’s no scientific evidence that conceiving identical twins is genetic. However, a woman is more likely to have fraternal twins if there are cases of twins in her immediate family, as it’s a consequence of superfecundation.

In conclusion, several characteristics help distinguish between identical and fraternal twins. By analyzing the factors described here, you can easily establish your case.

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.