types of twins

Twins 3544—Don Johnson 395 (Flickr.com)

The two most common types of twins are fraternal, also known as Dizygotic twins, and identical, also known as Monozygotic twins. “Mono’ means one, as in one egg, and “Di” means two, in this case, two separate eggs.

Fraternal twin births occur at a much higher rate than identical twins, about twice as often. Identical twins are born about 3 times out of 1000 births, whereas all types of twins combined are approximately 30 per 1000 births.

How the Different Types of Twins Happen

Identical twins happen when one egg is released from the ovaries, becomes fertilized, and then splits in two. Of the types of twins, this kind of twin pregnancy is much more rare and it is generally agreed that there isn’t any scientific explanation for why it happens, other than that the egg splitting is just a mistake made by the body.

The general consensus is that having identical twins has nothing at all to do with heredity, even though some families have these types of twins scattered throughout their family tree.

Identical twins are usually the same sex, but there have been times that they are not. This only happens when a male fetus drops a “y” chromosome, which causes the male to become a female, but this is very rare.

There are several different types of identical twins: mirror image twins, which is just like it sounds. They have opposite features such as one is right handed and the other is left handed, and may even have identical facial or body marks but on opposite sides of their bodies.

Polar body twins are actually half identical, they are from one egg that split, but the egg split before it was fertilized, and then fertilized by two different sperm after the split.

There have been cases of two sperm fertilizing one egg, and then the egg splitting afterwards. These types of twins are called semi-identical.

Conjoined twins are Monozygotic also (from one egg), but when the egg splits, it doesn’t completely separate. Conjoined twins are born attached somewhere because of this and sometimes share organs or limbs which makes separation difficult or even impossible.

Types of Twins – Your Odds Are Better For Having Fraternal Twins

On the other hand, fraternal twins occur when 2 eggs are released from the ovaries at the same time and both are fertilized. These types of twins are much more common and there are several reasons why. I like to say it’s caused by “over zealous ovaries” due to the natural formation of more than one egg at a time to be released by the ovaries.

This is more likely to happen, and thereby increasing your chances of having twins naturally, if you are older when you conceive, (at least in your late 30’s or in your 40’s), perhaps because your body has it’s own biological clock ticking away and is trying to take matters into it’s own hands?!

Also, it’s said that your chances of conceiving twins naturally is greater if you eat a diet that is very high in yams, which have been found to contain a natural chemical that stimulates multiple egg formation.

Another reason for becoming pregnant with fraternal twins would be that you have been taking fertility drugs to stimulate more egg production. By doing this, your odds of conceiving twins, or even triplets or more, may be higher.

Fraternal twins can have some unusual variants also. A woman may already be pregnant from one egg that’s been fertilized, and then become pregnant again from another egg that wasn’t fertilized the first time around. If a woman has sex with a different man which causes the second egg to be fertilized, the twins have different fathers and on occasion twins have been born of different races due to this.

Testing to Determine If Twins Are Identical

Even though fraternal and identical twins are the most common, there are a lot of factors involved in determining whether someone actually will have twins, and especially the types of twins that they could have.

Filed under: Having Twins