What Are The Different Types Of Twins?

We have all seen twins, some of us even have one! While whenever the word "twins" come to our mind, we think of fraternal or identical pairs. But did you know that there are many other types and subtypes of twins, including mirror twins, conjoined twins, and even semi-identical twins?

This is what we are going to dive in a little today.

Identical Twins: Identical twins or monozygotic twins come from a single fertilized eggs that splits into two embryos. These twin share the same genetic material, which is what leads to their identical physical traits and the same sex at birth. However, there are slight differences that can emerge due to environmental factors during pregnancy, including the space in the womb.

Fraternal Twins: They are also known as dizygotic twins. They come from two separate eggs, fertilized by two different sperm. They also share about 50% of their genetic material, and have unique physical traits, often different sexes at birth too.

Some researchers speculate about a third category: polar body twins or half-identical twins. This rare phenomenon might occur when an egg splits into two parts before fertilization, and each part is fertilized by a different sperm. Polar twins share chromosomes from their birthing parent but differ in those from the non-birthing parent, explaining why they might look similar yet not identical.

Mirror Twin: They are literal mirror of each other, with opposite hair whorls, birthmarks, and even dominant hands. This happens when an egg splits later than usual, around 7 to 12 days post fertilization.

Conjoined Twins: In cases where the egg splits after 12 days, incomplete separation can result in conjoined twins. They are physically connected, often sharing vital organs. Advances in surgical techniques have improved outcomes, including the possibility of separation in some cases.

Parasitic Twins: Parasitic twins occur when one twin stops developing and relies on the other for survival. The smaller twin, often incomplete, may appear as extra limbs or other appendages.

Semi-identical Twins: Extremely rare, semi-identical twins occur when one egg is fertilized by two sperm and then splits. Only two known cases of such twins have been reported.

A rare phenomenon called superfetation allows a second egg to be fertilized after a pregnancy has already begun, resulting in twins of slightly different ages.

In heteropaternal superfecundation, two eggs released in the same cycle are fertilized by sperm from two different people.

Twins with varying skin tones can occur due to unique combinations of genetic material, especially in biracial parents or cases of heteropaternal superfecundation.

Twin pregnancies come with higher risks, including: