Types Of Twins: Identical (Monozygotic), Fraternal (Dizygotic), And More

What Are The Different Types Of Twins? Hint: There Are More Than You Think

December 2, 2020 Updated March 26, 2021

Babies playing with toys indoors at a daycare.
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Whether you just found out you’re expecting or you’re trying to conceive, there are likely a million questions swimming around in your head. And if there are twins in your family, one of those questions probably centers on just that subject. Or maybe you’ve already discovered you’re having twins but it’s still too early for any additional testing. Either way, you’re here because you want to learn all about the different types of twins in pregnancy — and we’re here to give you the scoop.

Prior to this point, you may have thought there were only two types of twins: identical twins or fraternal twins. Actually, there are several more than that! And, not just that, there are several rare twin-specific events that can take place during a pregnancy that account for a few other lesser-known “types.”

So, prepare to become a learned scholar in all things twins.

How common are twins?

You probably know at least one set of twins (at least in passing). The truth is, twins are much more common these days than you might think. According to the Center for Disease Control and Prevention (CDC), the birth of twins has increased around 78 percent since 1980. That means that now around 34 out of every 1,000 births in the United States is a twin birth. This makes sense, given the many advances in fertility science over the years.

What are the “main” types of twins?

So, if someone asked you off-the-cuff what types of twins there are, you’d probably say fraternal and identical, right? And essentially, you’d be right — the vast majority of twins fall under these two types. There are other unusual variants and events, but we’ll get to those in a minute. For now, let’s discuss these two.

Fraternal (Dizygotic) Twins

The most common type of twins, fraternal twins develop when two separate sperm cells fertilize two separate eggs — often due to the mother releasing two eggs at once during ovulation. Both fertilized eggs then implant in the uterus and grow.

Fraternal twins only share about 50 percent of their DNA, making them no more genetically similar than non-twin siblings. They can be two girls, two boys, or a boy and a girl. They may look alike in the sense that many siblings (twin or not) look alike, but they are not identical.

Identical (Monozygotic) Twins

As you may have gathered, dizygotic stands for “two eggs” and monozygotic means “from one egg.” So, identical or monozygotic twins are formed when a single fertilized egg splits into two and then grows into two separate embryos. They may share the same placenta and amniotic sac, depending on when the fertilized egg splits.

As the name signifies, these twins are genetically identical — same DNA and therefore same genetic traits such as sex, blood type, eye color, etc. However, identical twins (which are much less common than fraternal) don’t necessarily look exactly alike. Environmental factors like the twins’ position in the womb can cause slight variations in appearance. Identical twins are born with identical DNA, but that DNA becomes more distinctive over time. Thus, each twin is still a “unique” individual.

How do you know if your twins are identical or fraternal?

Since identical twins don’t necessarily look exactly alike, it can be tricky to tell if they might actually be fraternal. If you’re preggers with twins, you may even want to know now which type of twins you’re having. But the only 100 percent accurate way to determine as much is to have the twins take a DNA zygosity test that compares the DNA profiles of each twin.

Are there other, more unusual types of twins?

Yep! Some you’ve likely heard of, others you may be hearing for the first time today. They include:

Mirror Image Twins

A type of identical twins, mirror-image twins develop when the fertilized egg splits more than a week after conception. Have you guessed from the name what this means? That’s right — the twins develop reverse asymmetric birthmarks and features that appear to be opposite reflections of each other, as if looking in a mirror. So, one twin might have a birthmark on their left cheek and the other will have the same birthmark on their right cheek. Or they’ll have cow-licks going in opposite directions.

Semi-Identical Twins

In this type of twinning, two sperm fertilize a single egg, resulting in three sets of chromosomes (also known as a triploid). That egg then splits. So, you end up with three sets of chromosomes but only two fetuses.

As embryos like this typically don’t survive, semi-identical twins are extremely rare. In fact, there are only two documented cases: The first was reported in 2007 by the Journal of Human Genetics, and the second reported in 2019 by the New England Journal of Medicine.

Polar Body or “Half-Identical” Twins

With semi-identical twins, the egg splits after being fertilized by two sperm. But what happens when the egg splits and then each of its halves meets (and gets fertilized by) a sperm? Theoretically, you get polar body or “half-identical” twins. Basically, this type of twin would have more matching genetic DNA than fraternal twins but less than identical twins. However, since no definitive tests exist to confirm polar body twinning, it essentially remains a theory.

Superfetation

It sounds like something out of a movie, but it’s possible to have twins conceived at different times. This occurs when an egg is fertilized during ovulation, but then a woman’s cycle is interrupted. When her cycle resumes, she’s already pregnant yet releases another egg — which also gets fertilized. Unlike with normal fraternal twins, the eggs are therefore from different menstrual cycles. In this exceedingly rare type of twinning, the twins might even be born on different days (perhaps weeks apart!).

Heteropaternal superfecundation

Also known as “bi-paternal” twins, superfecundation takes place when eggs are fertilized by sperm from different incidences of sexual intercourse. In such instances, the twins have the same mother but different fathers.

What are the types of twins caused by pregnancy events?

In rare cases, twins are the result of a complication or abnormality during pregnancy (as opposed to conception). These include:

Conjoined Twins

A type of identical twins, conjoined twins are the result of the incomplete division of the fertilized egg. Because of this, they join together at certain points of the body during pregnancy. They may even share limbs, organs, or tissue. It’s estimated that only about 1 in 85,000 births are conjoined twins.

Parasitic Twins

A type of conjoined twin, a parasitic twin pregnancy takes place when one fetus fails to develop completely and depends on the larger, stronger fetus. There are two types of parasitic twinning: fetus in fetu (when the parasitic twin develops inside the larger twin’s body) and acardiac (when the smaller twin is only a torso with or without legs whose heart is either malformed or missing).

Chimerism

When twins are conceived and one embryo dies in the womb, it may result in twin chimerism. What does that mean? Well, the surviving twin absorbs some of the cells of its deceased twins, giving that surviving twin two sets of cells (its own, and some of its twins).

Is twin pregnancy more painful?

Is carrying (not to mention delivering) two babies more painful than one? Surprisingly, the answer is no. Or at least not as much as you might imagine. Each woman has their own experience, but the general consensus is that childbirth isn’t a ton of fun. Most mamas carrying a double load complain more so about the pregnancy. They typically mention back pain, sleeping issues, and intense heartburn. Having said that, mamas carrying twins may experience fewer symptoms than some mamas pregnant with one baby. It’s all subjective. It is worth noting, though, that mothers carrying multiples have a higher rate of maternal anemia, prematurity, and postpartum hemorrhaging. Gestational hypertension is also higher in women having twins than women carrying one child. So, having twins doesn’t mean that you have a harder pregnancy or delivery, but it definitely comes with its own set of complications.

Are there tricks on how to get pregnant with twins?

If you and your partner want to see double during your delivery, there are few ways to board the twin train. Although twins are rare, there are many old wives’ tales you can try out to double your chances.

  • Bring on the cheese. When trying to have a baby, eat lots of cheese and milk products. There’s a protein in dairy that supposedly increases your chances of releasing more than one egg.
  • Eat sweet potatoes. If you’re already a fan of sweet potato pie, you’re halfway there. Eating this yummy starch may cause multiple births.
  • Chow down on some casava. It’s also known as yucca and is a common root vegetable served in Nigerian dishes. Nigeria has the highest twin birth rate in the world, and cassava could be the reason why. This shrub will put your ovulation into overdrive. Cassava’s phytoestrogen-filled peelings increase a woman’s chances of releasing two eggs.