Dizygotic (Fraternal) Twins: Facts You Should Know

Why Do Dizygotic Twins Occur and Which Are the Most Common?

Dizygotic twins in matching outfits, a baby boy and a girl
The most common kind of dizygotic twins are boy-girl. They occur 50% of the time. Eclisse Creazioni Art & Photography / Getty Images

Dizygotic means two (di) fertilized eggs (zygotes).  Dizygotic twins occur when two eggs are fertilized by two separate sperm. Dizygotic twins are also known as fraternal or non-identical twins. They are the most common type of twins. 

Unlike monozygotic twins (also known as identical twins), dizygotic twins do not share the same genes. Monozygotic twins share 100% of each other genes. Dizygotic twins share only 50%.

This is the same genetic similarity found between siblings conceived and born at different times.

Why Might Someone Conceive Dizygotic Twins?

Dizygotic twins may occur if two or more oocytes (eggs) are released in one cycle. If each is fertilized, dizygotic twins may result.

During IVF treatment, if two or more embryos are transferred, you may also get pregnant with twins (or more.) 

Fertility drugs are a common cause for dizygotic twins. Depending on which drug or fertility treatment is used, your odds of conceiving twins while taking fertility drugs ranges between 5 and 30%.

Medications like Clomid and fertility procedures like IUI and IVF are responsible for the majority of twin births in the United States.

But fertility drugs aren't the only cause for dizygotic twins. Your odds of conceiving twins may be higher if:

  • The mother is 30 years of age or older
  • The mother is taller than average
  • You have a family history of twins
  • You're overweight
  • You're African-American

Here is more information on your odds of conceiving twins:

Dizygotic Twins and Gender

Will you have boy-girl twins, boy-boy twins, or girl-girl twins?

Here are your odds:

  • Boy-girl twins are the most common kind of dizygotic twins, occurring 50% of the time.
  • Girl-girl twins are the second most common occurrence.
  • Boy-boy twins are the least common.

Dizygotic Twins and Their Amniotic Sacs and Placentas

Dizygotic twins typically have separate amniotic sacs and placentas. This is known as being dichorionic-diamniotic. (Sometimes called Di-Di for short.)

This is the most common set-up for dizygotic twins. Di-Di twins also have the lowest pregnancy risks. 

There are rare instances of dizygotic twins who share one placenta. In this case, each has his or her own amniotic sac. This is referred to as monochorionic-diamniotic (Mo-Di for short).

The risks are higher for twins sharing a placenta, due to the risk of twin-to-twin transfusion syndrome. The pregnancy may be monitored more closely.

Because two placentas can fuse over time, it can be difficult to identify via ultrasound whether there are two placentas or one later in the pregnancy.

 

For this reason, your doctor may send you for an ultrasound near the end of the first trimester, when it’s still possible to see the separation of two placentas.

Trizygotic Triplets and Quadrazygotic Quadruplets

If three eggs are fertilized by three separate sperm, this may result in trizygotic triplets.

If four eggs are fertilized by four separate sperm, you may get quadrazygotic quadruplets.

It is also possible for a mother pregnant with high-order multiples to have a combination of twins.

For example, triplets may start as nonidentical twins (or dizygotic twins). Then, one of those zygotes splits, leading to a set of identical twins (or monozygotic twins). All together, you would have two identical twins and one non-identical twin making up the set of triplets.

If you find this hard to understand, try this. Take two pens, one red and one black.

Draw a smiley face with the red pen, with a little baby hair on top.

Draw another smiley face with the black pen.

Now, from the black pen face, draw two lines down and out, each going to a separate new smiley face.

In one big circle, draw a line around one of the red faces and two of the black faces. Those are the triplets -- two identical twins (black pen) and one "sibling" twin (red pen).

However, the majority of high-order multiples are made up of nonidentical twins.

More about twins:

Source:

Racowsky, Catherine; Schlegel, Peter N.; Fauser, Bart C.; Carrell, Douglas T. Biennial Review of Infertility: Volume 2, 2011. Publisher: Springer; 1st Edition. (June 9, 2011)

Continue Reading