What Are Dizygotic Twins?

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's genes. Dizygotic twins share only 50%. This is the same genetic similarity as siblings conceived and born at different times.

Dizygotic Twin Conception

Dizygotic twins may occur if two or more oocytes (eggs) are released in one cycle. If each is fertilized, the result can be dizygotic twins. 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, the odds of conceiving twins range from 5% to 30%. Medications like Clomid (clomiphene) and fertility procedures like intrauterine insemination (IUI) and in vitro fertilization (IVF) are responsible for at least a third of twin births in the United States.

But of course, some people conceive dizygotic twins without using fertility drugs or treatments. Your odds of conceiving twins may be higher if:

• You are 30 years of age or older
• You have a family history of twins
  
• You're of African descent
• You're overweight
  
• You're taller than average

Factors Influencing Twin Conception

The odds of twins are highly affected by race, ethnicity, and even geographic location. According to research on twin births across developing countries, the highest rate of twins was found in central African countries while the lowest was found in Asian countries. European countries fall somewhere in the middle. However, even within these broad categories, there can be a lot of differentiation.

In the United States, rates of twinning have been decreasing after a few decades of rapid growth. Approximately one in 30 births or 32.6 per 1,000 pregnancies is a set of twins. For White Americans, the rate declined from 37 per 1,000 in 2009 to about 34 per 1,000 in 2018. For Black Americans, it was slightly higher at nearly 40 per 1,000. Hispanic people had the lowest rates in the U.S. with 24 per 1,000.

In Europe, the overall twinning rate is almost 17 per 1,000 births. The prevalence of twin births varies widely from country to country with Romania at the low end (9.1 per 1,000 births) and Cyprus at the other extreme (26.5 per 1,000 births). Other European countries with high twinning rates include Denmark (21 per 1,000 births) and Spain (20.6 per 1,000 births).

Most of Asia has very low twinning rates, often at or below 9 per 1,000 births. Vietnam has the lowest rate of 6.2 per 1,000 births. Most Central and South American countries also show low rates, primarily of below 10 per 1,000 births. However, the data in many of these Latin American countries are less comprehensive than in other parts of the world.

Western and Central Africa is home to the highest overall twinning rates, with rates of 18 per 1,000 or above. Some regions have a significantly higher prevalence, including Benin with a rate of 27.9 per 1,000 births. According to a 2020 study, "With a DZ twinning rate of 45 per 1000 live births, The Igbo-Ora Community in Southwest Nigeria has the highest dizygotic (DZ) twinning rate in the world."

Interestingly, these global variations in twinning rates are found for dizygotic twins. 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 3.5 to 4 per 1,000 pregnancies.

Dizygotic Twins and Sex

Dizygotic twins can be boy-boy, girl-girl, or girl-boy. 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 among dizygotic twins.

Amniotic Sacs and Placentas

Dizygotic twins typically have separate amniotic sacs and placentas, which is known as dichorionic-diamniotic (sometimes called Di-Di for short). This is the most common setup for dizygotic twins. Di-Di twins also have the lowest pregnancy risks among twins.

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

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, doctors may look for the placentas on ultrasound near the end of the first trimester, when it’s still possible to see the separation of two placentas.

Reducing the Risk of Conceiving Twins

There has been an increase in dizygotic twinning in developed countries across the world, in large part because of fertility treatments. For a few decades, twin rates went up and up, until the year 2000, when rates began to stabilize.

That's 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 because it is healthier for both the gestational parent and the fetus. It's not possible to completely eliminate the risk of twins during fertility treatment, but a few options are available.

Lowest Effective Dose

Using the lowest effective dose for fertility treatments can reduce the risk of twins. The highest risk for twins occurs during injectable fertility drug use (gonadotropins) with timed sexual intercourse or insemination. With this treatment, there is no control over how many follicles may develop or become fertilized.

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—follicles develop, the doctor may cancel the cycle and ask the couple not to have sexual intercourse. This can be difficult to hear, but it's important to remember that twin pregnancies can be risky.

Single Embryo Transfer

With in vitro fertilization, the doctor can control how many embryos are transferred back to the uterus by using single embryo transfer IVF (SET-IVF). 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 would "stick." But often, more than one embryo did, leading to increased rates of twin or multiple pregnancies.

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

Improvements in cryopreservation make single embryo transfer a viable option. Ideally, a prospective parent will have more than just one good-quality embryo. With frozen embryo transfer IVF (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 for future siblings, or they can be donated to another couple or to scientific research.

Trizygotic and Quadrazygotic Babies

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 person pregnant with high-order multiples to have a combination of twins.

For example, triplets may start as non-identical 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 need help visualizing how this would work: Take two pens, one red and one black. Draw a smiley face with the red pen. 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).

Identifying Dizygotic Twins

While an ultrasound technician can tell you how many babies you're carrying, it's significantly more difficult to determine whether the twins are monozygotic (identical) or dizygotic (fraternal).

That said, there are some signs that can indicate that your twins are dizygotic. Your twins are most likely fraternal if they are of different sexes or they have different blood types. If you're wondering how boy-girl twins could ever be identical, it can happen in some instances of rare chromosomal conditions.

If you have two boys or two girls, how will you know if they are fraternal or identical? If they happen to look really different, then you'll know. But sometimes, the only way to determine for certain is to complete DNA testing. If you're interested, you can talk to a pediatrician about DNA testing, or you can look into companies that offer DNA testing through the mail.