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

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Categories Developmental Geekery, Fraternal, Identical, Multiple Types, Science of MultiplesTags , , , 71 Comments

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.

wpid-Photo-Feb-11-2014-841-AM.jpg

Vocabulary

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 Canva.com. 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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The Top 10 Worst Reactions To My “It’s Twins” Announcement

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Categories Lists, Other people, PregnancyTags , , , , , , 12 Comments

I am 11 weeks pregnant with twins. This isn’t my first pregnancy. In fact, these will be my fourth and fifth babies. Since I’d announced three other pregnancies I foolishly thought this time wouldn’t be any different.

I had no idea that upon hearing “It’s twins” any filter or manners a person may have immediately go out the window. Here are the 10 worst reactions I’ve experienced:

  1. “Better you than me.”

    Why? Do you know something I don’t know?

  2. “I’m sure you’re stoked but I’d die!”

    This was said to me by a nurse practitioner at my pediatrician’s office. Shouldn’t she be easing my nerves? Shouldn’t she have wonderful advice and maybe other twins moms I could talk to? Shouldn’t she stop using the word stoked?

  3. “You’re going to need a new house”

    “You’re going to need a new car”

    Thank you for your concern but do you think that you’re sharing new information? I can assure you that the financial needs of 5 kids were some of my very first thoughts and fears.

  4. “Was this planned?”

    ummmmm… yes? I have always been an overachiever.

  5. “You’re going to HAVE to pump… give formula… get them on the same schedule… hire help.”

    I assume you’re basing this on your vast experience with twins.

  6. “My friend was pregnant with twins but she lost one at ___ weeks”

    Thanks. Like I wasn’t already worried about miscarriage or vanishing twin syndrome.

  7. “Welp, guess we won’t be seeing you next year!”

    Said a teacher at my son’s school. As she’s perusing the buffet I organized for a Valentines treat. No soup for you!

  8. “Maybe NOW you’ll get your girl”

    Because my 3 boys are so terrible?

  9. “Oh! Your poor poor boys”

    Siblings suck. So do big families. WTH?

  10. “You’re going to be HUGE!!!!”

    I know this is true, but I really don’t want to hear about it. Especially from someone wearing a size 0.

Not everyone’s reactions were awful. There are many sweet ones that stay with me when I’m feeling nervous about having 2 babies. The next time someone tells you they are expecting multiples please hug them, tell them they are the perfect mom for their babies, and remind them you’ll be there the whole time.

Elizabeth is expecting twins and is the mom to three amazing boys. She lives in central Texas.

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Which Came First

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Categories Parenting TwinsTags 27 Comments

And of course, I’m not talking about the chicken or the egg debate. It seems that many people (especially complete strangers) love to ask MoMs which baby was born first. From the MoMs I’ve talked to, I’ve learned that this question really, really bothers some mothers, while others think nothing of pointing out which child is Baby A.

My twins were born via c-section, just seconds apart. On their birth certificates, the time of birth is listed as 8:22am for both. So, when people ask me who was first, I usually just say, “They were both born at 8:22am.” If pressed for more details, I’ll usually share that Tiny is twin A, but what I feel like saying (especially if it’s a complete stranger asking) is, “Why do you care?”

I guess I’ve become more bothered by this question as Buba and Tiny have gotten older. When they were infants, it just seemed as though I was sharing a fact about their birth. But now that they’re older, it feels more like a label of their personalities. And I really want to avoid that as much as possible.

But what I’ve really been wondering about lately is what I’ll tell Buba or Tiny if they ever get around to asking that question. I suppose I could just answer honestly and matter-of-factly. After all, if they were singletons, there would be no possibility of keeping their birth order a secret. It just is what it is. But is it somehow different for twins? From reading numerous stories of twins (recorded in One and the Same: My Life as an Identical Twin and What I’ve Learned About Everyone’s Struggle to Be Singular), I get the feeling that it is.

How do you feel when you get asked the “which came first” question?  How do you answer it? And will you (or have you) shared birth order information with your multiples. Do you think it makes any difference whether they (or anyone else) know or not?

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