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’m looking for that ultrasound to show you guys. It’s in a box somewhere…) I didn’t need her to finish. Thanks to AP Bio (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.
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.
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.
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.
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.
Reading 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.
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!
TTTS can be very serious and put both your babies at risk. In cases of TTTS, the placental blood supply is shared unevenly between the babies, 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.”
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.
Sadia (rhymes with Nadia) has been coordinating How Do You Do It? since late 2012. She is the divorced mother of 7-year-old monozygotic twins, M and J. She lives with them and their 3 cats in the Austin, TX suburbs and works full time as a business analyst. She retired her personal blog, Double the Fun. She also blogs at Adoption.com and Multicultural Mothering.