Hey there, viewers! Twins and multiples are the subjects of much interest and attention. There’s
a sort of mystique surrounding them. While they have become more common in recent generations,
they’re still a relatively rare phenomenon. For every one hundred people you encounter,
only three will be a twin. Much fewer can claim to be a triplet.
Because of how rare they are, there are many misunderstandings about
twins. In today’s video, we’ll tell you what causes them. From what they are, to how they’re
formed, to the different types and more… Watch until the end to learn about all of them...
What are identical twins, and how are they formed?
Twins that look exactly alike are typically referred to as "identical" twins. However,
a more accurate term is "monozygotic." (maw-no-zigh-gottick) Monozygotic twins
share almost exactly the same genes.
This type of twin formation begins when one sperm fertilizes one egg.﻿
As the fertilized egg travels to the uterus, the cells divide and grow into a blastocyst.
In the case of monozygotic twins, the blastocyst then splits and develops into two embryos.
From there, two embryos grow into two babies.
Overall twinning rates vary across the globe. In the United States,
for example, the rate of twin births has declined in the last few decades,
and monozygotic twins make up a small percentage of the overall twin birth rate.
The rate of monozygotic twins is very consistent around the world. They account for about 3 to 5
in every 1,000 twin births. In some isolated populations, monozygotic twins seem to run in
the family. It remains unknown if genetics are at play, or if the environment has more to do with
it. It’s likely that something in the environment triggers the split, or it occurs randomly.
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Tell us quickly down below in the comments section!
What are the reasons for identical twins?
To put it simply, it’s a mystery. No clear explanation is offered
by science as to why a fertilized egg splits and develops into two embryos.
As technology improves, scientists are getting closer to finding answers.
One study used specialized computer software to capture photos of embryos developing.
It found that the embryo basically collapses, splitting the cells in half and dividing them
into two sets of genetic material that form as two separate fetuses.
While the discovery was important, it still didn’t pinpoint the reason for the split,
or explain exactly why identical twins happen. No genetic link has been identified.
Some theories have been proposed but not confirmed. These include an enzyme in sperm,
a mutation in the cells of the blastocyst, or the age of the egg after ovulation.
Identical twins are generally considered to be random and unexplained.
The mystery is part of their magic and mystique.
Contrary to popular belief, the chances of having monozygotic twins are most often not
related to your family history. If there is more than one set of identical twins in a family,
it’s probably due to luck or external environmental factors. Some genetic mutations
have been identified that increase the chance of monozygotic twins, but these are very rare.
In vitro fertilization, or IVF, it appears to increase the likelihood of having identical
twins.﻿ IVF research has given us some insight into how identical twins form. IVF embryos are
more likely than naturally conceived embryos to split into identical twins.﻿
A fertility doctor can transfer just one embryo—in hopes of reducing the risk of non-identical twins.
But identical twins may still occur, and more frequently than in the general population.
In normal embryo development, a fluid-filled cavity grows inside the embryo. This is known
as the blastocoel.﻿ Also inside the embryo is a collection of cells known as the inner cell mass,
which is a collection of cells that will eventually form the fetus.
During IVF, the blastocoel may collapse on itself.
While this typically destroys the embryo, sometimes the embryo survives, causing the
inner cell mass to split into two. The two inner cell masses lead to the development of twins.
The embryo is initially kept in an artificial solution in the lab.
While scientists have done their best to make this environment as close to natural as possible,
it isn't exactly the same environment as the inside of a woman’s reproductive system.
Perhaps this solution increases the risk of collapse.
Opinions differ on when to transfer the embryo into the woman’s uterus. It seems that
transferring the embryo later may slightly increase the odds of identical twinning.
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So what are the factors that increase the probability of twins?
Advancing age of the mother – Women in their 30’s
and 40’s have higher levels of the sex hormone estrogen than younger women.
This means their ovaries are stimulated to produce more than one egg at a time.
Number of previous pregnancies – The greater the number of pregnancies a woman has already had,
the higher her odds are of conceiving twins.
Heredity – A woman is more likely to conceive fraternal twins if she herself is a fraternal
twin, has already had fraternal twins, or has siblings who are fraternal twins.
Geography – Women in Africa have the highest rate conceiving twins,
while women in Asia have the lowest.
Assisted reproductive techniques – Many procedures rely on stimulating the ovaries
with fertility drugs to produce eggs and, often, several eggs are released per ovulation.
So, do twins share the same DNA?
It was once thought that identical twins shared the exact same DNA. But this isn’t true,
as every time cells split, there’s a chance of mutation. This starts as early as the first split.
This is one reason why identical twins are at an increased risk of congenital diseases.
After one split, individual cell masses continue splitting on their own. With every split,
there is another chance of mutation. Usually, by birth, the genes of twins are extremely similar,
but not identical. Over time, genetic dissimilarities continue to increase.
This is due to epigenetics—the way the environment changes how our DNA is expressed.
Some studies have found that slight DNA differences are more common in
older identical twins than in very young identical twins.
Spending more time apart in different environments is a likely contributor.
What other types of twins exist in the world apart from identical?
Fraternal Twins: The scientific term for fraternal twins — “dizygotic” (die-zai-gottick)— refers to
two fertilized eggs. Dizygotic twins happen when the birth parent’s body
releases two eggs at the same time. A different sperm will fertilize each egg.
Since fraternal twins are the result of different eggs and different sperm,
they share the same percentage of chromosomes as any other siblings, which is about 50%.
This is why they don’t look exactly alike, and are often a different sex at birth.
Half Identical Twins: A third type of twins exist called polar body or half-identical twins. After
the ovaries release an egg, the egg can split into two halves, the smaller of which is called
a polar body. This polar body contains all the chromosomes necessary to join with a sperm to
create a baby. But since it usually contains very little fluid, it’s often too small to survive.
It’s possible, though, that a polar body could survive and be fertilized. Meanwhile,
the larger half of the original egg could also be fertilized by a separate sperm.
The result? Polar twins. Polar twins share the same chromosomes from their birthing parent,
but they get different chromosomes from their non-birthing parent.
This is because they’re created from a single egg, but two separate sperm. For this reason,
they may or may not be the same sex at birth, and they may look very similar but not identical.
Mirror twins: Mirror twins are exactly what they sound like. These twins are actual mirror images
of each other. This means that their hair may naturally fall in opposite directions,
their teeth may grow in on opposite sides of their mouths or they may have birthmarks
on the opposite side of their bodies. They may also have different dominant hands.
In a typical identical twin pregnancy, an egg splits during its first week after fertilization.
But in a mirror twin pregnancy, the egg splits 7 to 12 days after it’s been fertilized.
This is long enough for the egg to have developed a right and left side.
Conjoined Twins: Conjoined twins are a rare occurrence in which the two siblings are
physically connected. Typically, conjoined twins are joined together at the chest or abdomen,
but this varies. Some conjoined twins are connected to a greater extent than others.
Most share at least one vital organ.
Though physically attached to each other,
conjoined twins are two individuals. They have unique thoughts and personalities.
Parasitic Twins: Sometimes, as twins develop in the uterus,
one twin becomes larger and more dominant. The other twin stops developing, and begins to
depend on their larger sibling. Known as parasitic twins, these twins are also physically conjoined.
However, the smaller twin is not fully formed and isn’t capable of surviving on their own.
This is because they’re usually missing vital organs, or a fully developed brain or heart.
In fact, you might not even recognize the smaller twin as a separate individual. This “twin” may
appear on the sibling’s body as a small lump, extra limb, or a second non-functioning head.
While identical twins might still be a mystery, there are other things that exist in this world
which were mysteries at first, but have now been solved thanks to science and research.
These include things like decoding our dreams, and what happens when you die.
Learn more of what we are talking about by watching:
What Happens When You Die.
Or...
What Our Dreams Mean.
Do you have twins in your family? What type are they? Let us know in the comments section below!