Fraternal Twins | Wonderful World of Dizygotic Twins?

Finding out you’re having a baby is one of the most wonderful times of your life. Discovering you’re having two babies means double the excitement.

You probably have a lot of questions about twins and the various types.

Let’s talk about fraternal twins and what makes these siblings so special.

Pregnancy lingo can be confusing and it gets even more complicated when you throw twins into the mix.

We’ll start with a bit of background to the language we use to talk about conception. 

Each month a single egg is released from the ovary. It moves down the fallopian tube, where it can be fertilised.

Conception occurs when male sperm enters the female egg, or ovum. 

The fertilised ovum is now called a zygote. It subdivides into a ball of smaller cells, called a blastocyst.

The blastocyst implants into the wall of the uterus and develops into a single embryo and placenta.

This is a single pregnancy with one baby.

Read on to find out what makes fraternal twins unique. 

What are fraternal twins?

Fraternal twins are the same as any other siblings, except they happen to be conceived in the same month and share their mother’s uterus at the same time.

They have the same biological mother and father.

Fraternal twins are also known as non-identical or dizygotic twins.

What causes fraternal twins?

Fraternal twins occur if two eggs are released from the ovary during the same ovulation phase. Then both eggs are fertilised by two separate sperm, resulting in two zygotes.

This is the origin of the term ‘dizygotic’ (‘di’ means ‘two’). Dizygotic twins is another name for fraternal twins. 

The two zygotes subdivide into two blastocysts. Both blastocysts implant into the wall of the uterus resulting in two separate embryos, each with its own placenta.

In a (dizygotic) fraternal twin pregnancy, each embryo is genetically unique.

Are fraternal twins genetic? 

Fraternal twins do tend to run in families, because the tendency for hyperovulation (when more than one egg is released) is a genetic trait.  

Studies show women who had altered regulation of a certain gene had higher levels of follicle stimulating hormone (FSH) in their blood.

FSH stimulates the growth of eggs in the ovaries. Higher levels of FSH cause the release of two or more eggs at once, resulting in fraternal twins. 

Women who have the gene that causes hyperovulation can pass it to their daughters, who are also more likely to have twins. 

Two eggs must be released, and two eggs must be fertilised for fraternal twins to occur.

Women carry the gene that causes twins, and women ovulate. There’s nothing in sperm that causes twins.

The only role played by the male in conceiving twins is the presence of enough healthy sperm to fertilise the two eggs waiting in the fallopian tubes.

Can fertility treatment cause fraternal twins? 

Medications used to increase the chances of becoming pregnant during fertility treatment stimulate the ovaries to produce more eggs at once.

Unfortunately there’s no way to gauge how an individual woman will respond to the medication.

Hyperstimulation of the ovaries is a side effect of fertility medication. It might result in two eggs being released during a cycle.

If both eggs are fertilised the result will be fraternal twins. 

The incidence of fraternal twins increases with age.

Women over 35 are more likely to release more than one egg during ovulation. If both eggs are fertilised she will have fraternal twins. 

Do twins skip a generation? 

You may have heard that twins skip a generation. It’s a myth.

The explanation lies in the fact that women are responsible for carrying the gene that causes twins.

Men can inherit the gene from their mother and pass it to their daughters. Their daughters would then possess the gene that regulates FSH and be more likely to conceive twins. 

Can fraternal twins be identical?

Fraternal twins aren’t identical. Each embryo develops its own placenta and surrounding amniotic sac and fluid.

They develop separately, and not necessarily at the same rate, in utero and after birth.

In some cases, they can be so similar people might assume they are identical.  

On the other hand, they might look nothing alike, and can even have different eye colour and hair colour.

Fraternal twins also have different personalities and can be of varying size and stature. 

Are fraternal twins the same sex? 

Fraternal twins can be the same sex or be a male and female pair.

It’s possible they’ll look very similar, especially if they are same sex twins. 

The most commonly occuring fraternal twins are boy-girl (50%) followed by girl-girl. The least common type of fraternal twins is the boy-boy pair.

The incidence of fraternal twins is much higher than identical twins. Identical twins are always the same sex.

What is the difference between identical and fraternal twins?

The terms ‘identical’ and ‘fraternal’ don’t describe what the twins look like, but actually how they are formed.

The easiest way to remember the difference between identical (monozygotic) twins and fraternal (dizygotic) twins, is the number of eggs involved. 

Identical twins develop when one sperm fertilises one egg but the zygote splits in two.

Fraternal twins develop when two separate eggs are fertilised by two separate sperm resulting in two zygotes. 

Another difference between identical and fraternal twins is their genes. 

Identical twins have the same DNA and genetic origins because they come from one zygote that splits in two.

They share the same placenta and usually develop at the same rate during pregnancy. They share all of their genes and are the same sex.

You can read more about identical twins here. 

Fraternal or dizygotic twins are no more alike than other siblings. Although they share 50% of their chromosomes, they have unique DNA and can develop differently. 

What is the difference between paternal and fraternal twins?

‘Paternal twins’ is another name for identical or monozygotic twins.

It is a non-scientific term, interchangeable with ‘maternal twins’, and based on inherited characteristics from either the father or the mother.

Although paternal twin appear identical, often they have individual distinguishing features, such as moles or freckles.

As they grow, they might develop style and body language that’s unique.

Even though they share the same DNA, external environmental factors influence their personality and development.

What are the 3 types of twins?

Fraternal twins

These are the most commonly occurring twins.

The rate of fraternal twins varies according to demographics such as age and country of origin and is approximately 22.8 per thousand in the world. 

Identical twins

The rate of identical twins remains constant at approximately 4 per thousand.

Most identical twins happen by chance and they can happen to anyone.

Of all twins, about one-third are identical and about two-thirds are fraternal.

Polar body twins

The least commonly occurring twins are half identical twins (semi identical twins), also known as polar body twins.

Polar body twins are the result of a mature egg splitting in two before fertilisation occurs.

Both halves have everything required to make a baby but often one half is smaller than the other.

The smaller half is called a polar body and usually has very little fluid (cytoplasm) inside.

Although this makes it unlikely to survive, there are instances where the polar body survives long enough to be fertilised by a male sperm while the other, larger half is fertilised by another sperm. 

Semi identical twins are extremely rare and there’s a very small incidence around the world.

They might be same sex or different sex and, although they appear very much alike, they aren’t identical. 

Polar twins share approximately 75% of their DNA.