Understanding the Different Types of Twins

By Angelita Williams

For the most part, when the majority of us think of twins, we think of identical twins and fraternal twins. While these two types of twins are by far the most common multiples we see, there are several other types of twins that exist within these categories. There are many misconceptions about multiples in the general public. Whether you are the parent of multiples, you have multiples somewhere within your family or friend group, or you are just hoping to start a family of your own, it is important to take an educated and straightforward look at the different types of multiples that occur. The following are the three primary types of multiples that can occur and then a description of several rare multiple types within these three primary categories:

Fraternal/Non-Identical Twins

Fraternal or dizygotic twins are commonly known as non-identical twins or dissimilar twins. These types of multiples occur when two fertilized eggs are implanted in the uterus wall at the same time. These eggs are fertilized independently of one another by two different sperm cells, but are both implanted in the uterus. These twins can be either both female, both male, or one female and one male. Essentially (at least scientifically speaking) fraternal twins are siblings that happen to share a birthday. Just as with any other siblings, fraternal twins have a very small chance of sharing a chromosome profile. These twins can look very similar or they can look very different from one another. Dizygotic twinning is determined solely by the mother. There is no scientific evidence that supports the father having any cause for the release of more than one egg from the mother. Fraternal twins are more common among older mothers, with cases doubling in mothers over the age of 35.

Monozygotic/Identical Twins

Monozygotic twins occur when a single egg is fertilized to form one zygote, which later divides to create two separate embryos. This process is believed to be a completely random and spontaneous event. The incidence of monozygotic twins is about three in every 1000 deliveries worldwide. Monozygotic twins are more commonly known as identical twins to the general public. These multiples are always the same sex and are almost genetically identical to one another. Interestingly, identical twins do not have the same fingerprints. Even within the small environment of the womb, people encounter different aspects of their environment, creating small differences and variations between them.

Semi-Identical Twins

This form of twinning is extremely rare, but is another type of twins that many people do not know about. Semi- or half-identical twins are twins that inherit the exact same genes from their mother, but different genes from their father. There is actually very little known about the science behind semi-identical twinning. For the most part, parents will not know if they have semi-identical twins or identical twins without having genetic testing performed.


This guest post is contributed by Angelita Williams, who writes on the topics of online courses. She welcomes your comments at her email Id: angelita.williams7 @ gmail.com.