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
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 percent of each other genes. Dizygotic twins share only 50 percent.

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 of 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 percent.

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:

What Location and Ethnicity Has to Do With Your Odds for Twins

The odds of twins is highly affected by ethnicity and even location.

Those of African ethnicity are most likely to have twins, while Asians are least likely. Europeans fall into the mid-range. 

However, even within these broad categories, there can be a lot of differentiation.  

For example...

  • In the United States, the twinning rates in 2003 were found to be 15.8 per 1,000 pregnancies. For Caucasian Americans, the rate was 15.7 per 1,000, while for black Americans, it was higher at 17.2 per 1,000. 
  • In Europe, the overall twinning rate in 2003 was 16.4 per 1,000 births. In Luxembourg and Portugal, the twinning rate was 11 in 1,000. Compare that to the twinning rates in Denmark, Greece, and the Netherlands where it was 20 per 1,000 births—almost twice as high. 
  • Nigeria has an overall high twinning rate, with an overall dizygotic twinning rate of 40 per 1,000 births. Still, variations are found in different regions. In Western and Eastern Nigeria, twinning rates were between 33 to 66.5 per 1,000 births. In Nothern Nigeria, rates were 19.4 per 1,000 for specific groups.

These variations are found for dizygotic twins. But these differences by region and race are not found for monozygotic twins, or identical twins. The rate of identical twins is pretty much constant across all regions, occurring in approximately 4 per 1,000 pregnancies.

 

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 percent 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.

    Reducing Your Risk of Twins When Using Fertility Treatments

    There has been an increase in dizygotic twinning in developed countries across the world, and this in large part occurred because of fertility treatment use. For a few decades, twin rates went up and up, until the year 2000, when rates finally started to drop slightly. 

    They reached a peak and started to go down because as fertility treatment technology has advanced, reproductive specialists have found ways to reduce the number of multiple pregnancies. The goal of infertility treatment should always be one healthy baby, one at a time. 

    It's not possible to completely eliminate the risk of twins during fertility treatment, but a few options are available.

    Using lowest effective dose for fertility treatments: The highest risk for twins occurs during injectable fertility drug use (gonadotropins) along with timed sexual intercourse or insemination. There is no control over how many follicles may develop or become fertilized. 

    However, when doctors use just enough fertility medication to stimulate just one or two follicles, the odds of multiples can be reduced. Of course, if there are two follicles, the risk for twins is higher than if there is just one follicle. 

    Also, if more than one—or a few to several—follicle develop, the doctor can "cancel" the cycle and ask the couple not to have sexual intercourse. 

    Single embryo transfer IVF (SET-IVF): With in vitro fertilization, the doctor can control how many embryos are transferred back to the uterus. When IVF was newer and less effective, transferring two to four embryos at a time wasn't uncommon. The hope was that at least one will "stick." But sometimes, more than one "stuck." 

    Now, with young and good prognosis patients, single embryo transfer is an option. In this case, only one good quality embryo is transferred. There is still a very low risk of identical twinning. But the higher risk of dizygotic twinning is eliminated. 

    Frozen embryo transfer IVF (FET-IVF): Improvements in cryopreservation make single embryo transfer a viable option. Ideally, a couple will have more than just one good quality embryo. But what happens to the embryos not transferred? 

    With FET-IVF, the "extra" embryos can be frozen for now. Then, if pregnancy doesn't occur, a future FET-IVF cycle can be tried. 

    If pregnancy does occur with the first embryo transferred, you can keep the cryopreserved embryos "on ice" for future siblings, or they can be donated to another couple or to scientific research. 

    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). Altogether, 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 non-identical twins.

    Source:

    Hoekstra C1, Zhao ZZ, Lambalk CB, Willemsen G, Martin NG, Boomsma DI, Montgomery GW. “Dizygotic twinning.Hum Reprod Update. 2008 Jan-Feb;14(1):37-47. Epub 2007 Nov 16.

    Pison, Giles; Monden, Christiaan; Smits, Jeroen. “Twinning Rates in Developed Countries.Population and Development Review. Wiley Periodicals, Inc. 41(4): 629–649 (DECEMBER 2015).

    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)