Monday, October 5, 2015

Twin DNA Testing

I remember my first ultrasound at 10 weeks.  Like any first-time moms, it was exciting but scary as we heard the following:

Two sacs.  Two placentas.  Two babies.

Um, what?

Not only are you getting two for the price of one (and NOT REALLY) but moms of multiples also get a free lesson in human reproduction that we never learned in high school health class.

What’s that thing that looks like a spider web?  That’s the membrane separating the two sacs.

Are you sure that second heartbeat isn’t mine?  It would only be yours if you were running a marathon.

Explain to me how this happened again??
One of the best parts of being pregnant with more than one human is the countless ultrasound photo shoots!
My scenario – two babies, two sacs, two placentas - is medically known as di/di. But, there’s also mo/di and mo/mo and for some reason, why does it all remind me of that song from The Sound of Music? (Oh that’s right, the Von Trapp family had, like, 75 children.)

Here are the possibilities:

Mo/mo (monozygotic/monozygotic): Two babies sharing a sac & a placenta

Mo/di (monozygotic/dizygotic): Two babies, two sacs & sharing one placenta

Di/di (dizygotic/dizygotic): Two babies, two sacs & two placentas

Identical: From one egg, boy/boy or girl/girl 

Fraternal: From two eggs, boy/boy or girl/girl or boy/girl*
(*Reminder: Boy/girl twins can NEVER** be identical, no matter how times people will ask about your son and daughter.)
(**Except in an extremely rare case:  One identical twin boy drops or loses a Y chromosome, resulting in a single chromosome, or an XO chromosome. Without a Y chromosome, the twin becomes a girl with a single chromosome, and will have Turner's Syndrome.  There have only been a few documented cases so it's fairly safe to say they can NEVER be identical.)

When my fraternal girls, Taylor and Madison, were born, they couldn’t look more different!
Yep, they definitely came from two eggs! I think…
Until, they started growing rolls and double chins – then, they began to look a lot more alike.  Same blonde hair.  Same green eye color.  At each well visit, they always clocked in at the same height, same weight.

I’m pretty sure…Taylor is on the left, Madison is on the right?
And I often heard – from strangers and family - “They must be identical, right? Because I certainly can’t tell them apart.”  Yes, I’m sure! I’m their Mom! Of course, they are fraternal. Right?


Di/di twins can be identical if the egg split within the first few days of conception.  Four or more days after, if the egg splits, the babies will share a placenta – Mo/Di.  Eight or more days later, if the eggs split, the babies will share a placenta and a sac – Mo/Mo.  (I told you we’re giving a reproductive lesson here! And since I was an English, not a Biology major, here’s a handy dandy visual.)

I don’t understand any of this medical jargon but the pictures speak for themselves.

We wanted to find out for sure – did I ovulate one egg that split or did I ovulate two eggs? 

Several of my fellow twin mommies from BabyCenter and Instagram did similar tests and recommended Proactive Genetics.  For $100, we swabbed the girls’ cheeks, and sent away for the results. 

Less than two weeks later, we received the news: Identical.  

My husband knew they were identical.  I’m still in shock!
Mind blown.  It was like finding out we were pregnant with twins again!  Which now, the likelihood of that happening again, is very rare.

There is so much information out there but one of the easiest and most organized article for me was this post on Huggies’ Australia site.  Basically, it says identical twins is luck or random.  There is no genetic or family history with identical twins.  Whereas, fraternal twins can be hereditary.  Chances are, if your grandmother or mother had fraternal twins – she released two eggs during her ovulation cycle, which is a trait that could be passed down to you.  (Here’s another helpful online article about how to tell if twins are identical or fraternal.)

There are even more possibilities, like mirror-image twins or superfecundation twins, where there are two daddy’s.  Again, I’ll leave it to the experts at Multiples of America to explain that one.

Identical twins does not mean we will both look at the camera at the same time.  Joke’s on you, Mom.
Does it really matter if your little ones are identical or fraternal? Not really.  It is said, God forbid, if one identical twin gets a disease or medical condition, it is likely the other twin will get it.  But, at the end of the day, it doesn’t change a thing.  However, it did matter to us and I know it will matter to the girls as the get older.  Multiples are unique no matter how you slice or dice them and parents of twins, triplets and more all know their children will always have each other’s backs – no matter how they were made.


  1. This is so interesting, thank you for posting! I knew some of this (I am a mom of 2-year old b/g twins), but definitely didn't know all of this. Congratulations on your adorable girls. So precious.

  2. We have di/di identical twins too! When we were waiting for testing and people asked I always said we weren't sure yet. For some reason they could not fathom how it was possible not to know in utero. I will now refer them to this post, thank you!

  3. I feel like I could have written this! Great post, not many people know about the possibilities and I'm glad I found an article describing that twins in 2 sacs and 2 placentas can be identical. I feel like doctors should inform their patients of this chance as well. We used the same company for the DNA test and got the same letter!

  4. This is the most fascinating and well organized twin article I've ever read!

  5. I'm the mom of mo/mo boys. Very well explained. So many people I talk to want to understand this but their heads spin when I try and explain. Very well done.

  6. Great post. I have mo/di identical girls and it is tough explaining this to people. I get into the differences and people always say that they never knew twins were that complicated.

  7. This is exactly what happened to us! Di/di boy twins that we were told were fraternal our whole pregnancy. I had no idea they could even be identical until I started researching! They looked so different to us when they were born and as they grew. But to everyone else, they looked exactly the same! We would get asked every time we were in public if they were identical and we would say "nope! They're a fraternal!!... We think". We also used proactive genetics and found out on Christmas last year that they were in fact identical. Great post! I know I've loved learning more about all these different possibilities of twins from my research!