Since the beginning of time, people have always had a fascination with twins in the family. Twins are treated as “special” and “good omen” to families’ fortunate enough to have them.

Nowadays, with so much advancement in fertility science, it is becoming more common to see more families having twins, triplets (three babies), quadruplets (four), quintuplets (five), sextuplets (six), septuplets (seven) and octuplets (eight babies)!

Today’s article will focus on different types of twins, and mention some rare types of twins.

Identical twins

Identical twins are formed when one fertilized egg splits into two equally, normally within the first week of fertilization. They are scientifically termed as monozygotic twins.

Since both parts come from the same egg and sperm, each twin has the same set of chromosomes and they are born identical.

Identical twins have the same sex, and have the same genetic characteristics like eye colour, height and hair.

There may be very slight differences in the appearance of identical twins. This may be caused by one twin taking up more room in the womb.

Fraternal twins

Fraternal twins, scientifically termed as dizygotic twins, are formed from two fertilized eggs. This means the mother releases two eggs at the same time, and both eggs end up being fertilized by different sperm.

Coming from different eggs and sperm, the twins only share about 50 percent of their chromosomes like other siblings.

Fraternal twins do not look the same, and can be of the same or different sex, e.g., a boy and a girl.

Polar body twins

Some fraternal twins may look identical, and are identified as polar body or half-identical twins.

In the case of polar body twins, one unfertilized egg splits into two unequal eggs. The smaller half is called a polar body. The polar body egg can develop into a body if it gets fertilized. However, as a polar body egg has very little fluid, it normally does not survive.

However, if a polar body egg survives and gets fertilized, while the other bigger egg is fertilized separately, the result would be polar twins.

Polar twins only have the mother’s chromosomes. They can be of the same or different sex.

Mirror image twins

These type of twins are formed when the egg splits 8 – 12 days after fertilization. The single embryo would have already developed a left and a right side by this time.

So when the egg splits, the twins are identical and mirror images of each other.

A simple example of mirror image twins is one twin may be left handed whereas the other twin may be right handed. Or one twin may have teeth coming in from the left and the other’s teeth come in from the right.

Conjoined twins

Conjoined twins form when a fertilized egg does not completely split into two. This phenomenon sometimes happens when the egg divides 12 days or more after being fertilized. Another possibility is the egg did split but somehow fused back together.

Conjoined twins are commonly fused together at the chest or abdomen. The case for separation becomes extremely difficult when the twins are found to share one or more vital organs.

These type of twins are actually two individual personalities who think independently. Unfortunately, conjoined twins often have very short lifespans.

Other rare twins

Apart from the types of twins mentioned above, there are other rare sets of twins, e.g., parasitic twins, semi-identical twins, boy/girl monozygotic twins, and even twins with different fathers. As these are rare cases, we will not go into detail about them as the scientific explanations are fairly lengthy.

In conclusion: we hope today’s article has given our readers some further insight into the diverse world of twins, identical or otherwise. Please note – the contents of this article are purely informative and not to be taken as any form of medical advice.


By Aaron
9th October 2020 22:30

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