Identical Vs Fraternal: What Your Doctor Didn’t Explain About Your Twin Ultrasound

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My Story

(or skip to the basics or the science)

Ultrasound of identical twins at 7 weeks. You can't see the membranes in this image.

I was 7 weeks pregnant when I had my first ultrasound. The doctor pointed out the shared outer sac (chorion) and the two distinct inner sacs (amnions). I didn’t need her to finish. Thanks to Advanced Placement Bio class in high school (embryonic development) I knew I had a miracle in my womb: identical twins. Once we’d called everyone we needed to share the good news with, I hit Google, and quickly concluded from their monochorionic/diamniotic (mono/di) state that my little ones had split from a single cluster of identical cells somewhere between 3 and 9 days after conception. I’ll tell you how I made the calculation in a little bit.

The Basics

Most people don’t know a whole lot about twins or higher order multiples, and are intrigued by them. Folks I run into are usually aware that there are two basic types, identical and fraternal, but often don’t know precisely what the difference is. Part of this comes from the term “identical.” In casual English, “identical” means “exactly the same,” and so people often assume that identical twins should look alike, act alike, and think alike. This assumption often gets extended to fraternal twins, in that they should look different, act differently, and think differently.

I don’t argue with people about whether my children look enough alike to be “really” identical, and instead give them a quick science lesson. You’d be surprised how many medical professionals, even obstetricians, don’t remember the science of twinning they covered in the depths of college or medical school, and therefore jump to possibly incorrect conclusions about whether a set of twins is identical or fraternal. Next time you need to explain the distinction to someone, feel free to use the visual aids below.

TWINS! Understand the basics with this clear primer. Click To Tweet

The Science

Identical multiples grow from the same fertilized egg and therefore have basically the same DNA. Fraternal multiples come from different fertilized eggs, and therefore basically share 50% of the same DNA, as do siblings conceived by the same parents at different times. Sharing a DNA template makes it likely that identical siblings will look very much alike, but DNA doesn’t predict everything.

My daughters, for instance, share their DNA, but have noses of different shapes and different hairlines, due to developmental differences that don’t appear to have a genetic basis. They’re also different heights, likely because one is a pickier eater than the other and because dysphagia related to macroglossia (trouble swallowing because her tongue was too big for her mouth) meant that she ate less than Sissy after she weaned.



Before I go much further into the science, let’s talk about the terminology we’ve been using.

Basic terminology to describe babies in the womb.

So, the embryo is inside the amnion, which is in turn inside the chorion. The umbilical cord traverses the two membranes to connect the embryo to the placenta, which collects nutrition from mommy for baby.

Twins in the Womb

Now let’s talk twins.

Monozygotic twins are identical ones. They started from a single zygote. (Mono means one.) Dizygotic twins are fraternal ones. They started from two zygotes. (Di means two.)

Monochorionic/monoamniotic (mono/mono) twins are monozygotic twins who share a single amnion and a single chorion.

Monochorionic/diamniotic (mono/di) twins, like my daughters, are monozygotic twins who have separate amnions and share a single chorion.

Dichorionic/diamniotic (di/di) twins are monozygotic or dizygotic twins who have separate amnions and separate chorions.

I try to make this clearer in the image below. With one egg and sperm, you can get one baby… or two babies who are mono/mono, mono/di or di/di. With two eggs and two sperm, you’ll always get di/di twins.

The different membrane configurations possible for twins in the womb. The chorion is on the outside, the amnion on the inside.

So here’s the trick. In the image above, you can’t tell the difference between the identical di/di twins and the fraternal di/di twins. And neither can the ultrasound tech. So, if you have di/di twins, chances are good that they’re fraternal, but you just don’t know for sure.

If you have di/di #twins, chances are good that they're fraternal, but you just don't know for sure. Click To Tweet

Reader Noura I was kind enough to share ultrasound images of her di/di identical twins, whose ultrasounds look just like those of fraternal twins. Remember, the mono-di stuff refers to the membranes around the babies, and not the numbers of eggs and sperm.

Dichorionic diamniotic identical twins at 6 weeks gestation.

di di twins 1st trimester

Reading the Ultrasound

What you can know about your twins zygosity from the ultrasound

So, in my little chart above, I had to note that there are extraordinarily rare cases of boy/girl identical twins, but this is a teeny tiny proportion of the population. If you ran across such a pair, you’d recognize them from the news. So, please, just assume that boy/girl twins are fraternal (dizygotic) or that one had a sex change. Either way, it’s not polite to ask. Girl/girl twins and boy/boy twins can be fraternal or identical.

Timing of Monozygotic Twin Split

Here’s a fun fact. The arrangement of amnion and chorion can tell those of us with identical twins when they split apart!

The membranes on your ultrasound tell you something about your identical twins schedule for splitting.

TTTS can be very serious and put both your babies at risk. The placental blood supply is shared unevenly, meaning that one has more than his or her share of nutrition and oxygen, the other less than his or hers. Many obstetricians will closely monitor mothers expecting twins to watch for TTTS. While it’s almost unheard of with fraternal twins, reader Halie H. wrote to us to say, “My di/di fraternal (boy/girl) twins’ placentas fused. They were born with one failed and one really really red placenta; they were sent off to be studied as an example of TTTS in fraternals.”

In #TTTS, the placental blood supply is shared unevenly between twins, putting both babies at risk. Click To Tweet

I’m not an expert on this stuff, but I do love genetics and studied it in college (although I ended up switching away from a biology major junior year). If you have additional questions, I’ll do my best to answer them.

Before I sign off, I need to give a big old shout out to I have been planning to write this post for years, but not having an artistic bone in my body, knew that I couldn’t do it justice without an illustrator. Thanks to the free online graphic design tool, Canva, I was able to create the graphics I’ve included in this post.

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Identical Doesn’t Mean Identical

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If I didn’t know from an early ultrasound that my daughters were identical twins, I would have just assumed that they were fraternal. I’ve known fraternal twins and different aged siblings who’ve looked more alike than J and M do, at least to my eye. We still get asked if they’re identical occasionally, but most people are surprised to learn that they are.

J and M, posingAs M likes to point out, her widow’s peak hairline makes her face heart-shaped, where J’s is oval, thanks to her ruler-straight hairline. J is built like a soccer player, all lean muscle and power, while M has a typical dancer’s frame, birdlike and flexible.

J inherited Daddy’s single dimple, but M didn’t. J’s cowlick is profoundly untameable, while I can get M’s hair to hang down nicely with a little effort. On the other hand, I can part J’s hair in the middle, put it in pigtails, and have her hair stay generally well-behaved all day. M’s part, on the other hand, clings stubbornly to its location. It’s stronger than any combination of hair elastics, gel, bobby pins and effort I’ve been able to come up with. I’ve stopped fighting it, even if it does cause her pigtails to noticeably differ in thickness.

M was born with a facial cleft, which hasn’t needed any surgery so far. I hope it stays that way. Rather than the more familiar cleft palate, her frontonasal dysplasia is higher up in her face, and is the cause of her defined widow’s peak. It also causes her eyes to be more widely set than her sister’s and impacts the symmetry of her nose. She hasn’t had any complications from her condition, so we don’t spend a whole lot of time thinking about it. On the rare occasion that a child asks why M’s nose is funny or little, I say that it’s so we can tell her apart from her sister. That answer has always satisfied diminutive inquisitors.

Every now and then, though, I catch a glimpse of Sister in the face of one of my daughters, and the sameness makes my breath catch.

Adult identical sisters hold 4-year-old identical sisters.I’ve come to enjoy the opportunities I get to share the science of twinning with strangers. I’ve learned to explain in a few words that identical twins are identical(ish) at the level of DNA, but are otherwise completely distinct people. Still, I’m taken aback every time I participate in the following exchange:

Stranger or acquaintance: Are they identical?
Me: Yes.
Stranger or acquaintance: No, they’re not!


Do you know whether your multiples are identical or fraternal? Does it make any difference?

Sadia is a business analyst living in El Paso, TX. Her twin daughters, J and M, will be turning 6 next month.

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Zygosity: Do you know? Do you care?

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I spent part of the weekend attending workshops about twins and twinning. (I intended to post something immediately when I got home, but life got in the way and I’m a few days late). I found it quite fascinating to hear about what scientists know and don’t know about how twins are created. It was equally interesting to discover what twins have taught us about the joint influence of genetics and environment. One of the recurring themes of the workshops focused on different types multiples.

Here’s a quick review:

  • Monozygotic (MZ) multiples (can be twins or higher-order multiples) were created when one fertilized egg split to form two or more embryos.
  • Dizygotic (DZ) or trizygotic (TZ) multiples were created when two or more eggs were fertilized.
  • Triplets and higher-order multiples can be any combinations of monozygotic and di/trizygotic.

Monozygotic multiples are commonly known as “identical” and di/tri/quadzygotic are known as “fraternal.” These terms can be somewhat misleading as they suggest that monozygotic multiples are the same in every way. The truth is that the moment the egg separates, the two eggs cease to be identical and they are then influenced by different internal and external conditions. As a mother of monozygotic twins, I can tell you my daughters look very similar, but they are unique individuals and should be treated as such.

Anyway, back to the workshops, I learned a lot of interesting trivia about multiples. Did you know:

  • that monozygotic twins have more variation in birth weight than dizygotic twins, but by age 10 monozygotic twins are closer in weight and height than dizygotic twins?
  • there are more sets of female monozygotic twins than male sets, and more sets of female conjoined twins than male sets?
  • for most cancers, the risk of getting cancer isn’t any higher even if your twin has it
  • even though monozygotic multiples aren’t supposed to run in families, there are some cases where it seems to recur in families or to occur more often than expected in families with dizygotic multiples
  • language delays and learning disabilities seem to be more common among multiples
  • dizygotic twins may run in families and may be passed down by both men and women
  • there are mirror image twins where some traits are opposites (one is left-handed and one is right-handed, one has hair that parts on the left and one on the right, etc)

Since dizygotic twins run in both my family and my husband’s, we assumed that the twins we were having would be dizygotic. When they were born, the doctor said he thought they might be monozygotic and sent the placenta for testing to confirm his theory. If he hadn’t taken that step, we may have continued to assume they were dizyogtic even though they look very similar. I’m not sure if having that information makes a difference to us, at this point. Many families go for years without knowing the zygosity of their multiples, and apparently 75% of them are right about their assumptions.

Do you know for sure the zygosity of your multiples? How did you find out? Why is it important for you to know?

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