Fraternal or Identical? I Don’t Know!

The learning curve when it comes to a twin pregnancy, twin birth and twin parenting is hella steep. It’s pretty much vertical.

One of the things I learned quickly was that my doctors couldn’t tell me if my twins were identical or fraternal. This was confusing.

For the past two years, when folks have asked me if I conceived my twins naturally, if I delivered them vaginally, if I’m breastfeeding them and if they’re good babies, they’ve also asked me if they’re identical. So my answers have been yes, yes, yes, define “good” and I do not know.

People are often confused.

Twinguistics 101

Identical twins come from the same fertilized egg that splits into two. They have the same DNA because they came from the same egg. Contrary to what a surprising number of folks seem to think, twins of different sexes cannot be identical. (That may seem obvious, but my fellow twin parents with boy/girl sets are asked this at an alarming rate, even by healthcare professionals.) If you really want to split hairs, there are extremely rare cases (rare as in 2 so far “discovered”) of semi-identical boy/girl twins.

Fraternal twins are two eggs and two sperm, so genetically speaking, they’re as related as any set of full siblings. The mother’s body either accidentally dropped two eggs during ovulation (oops!), or multiple eggs may have been implanted or stimulated during fertility treatments.

Isn’t it obvious from ultrasounds which type of twins you have?

Twin pregnancies are higher risk than singleton pregnancies (and the risk of incorporating the word “singleton” into your daily vocabulary is extremely high if you are experiencing a multiples pregnancy). Higher risk pregnancies require more frequent ultrasounds, so twin moms get to see their twins in black and white fairly often.

The twins are identified as “A” and “B,” with A theoretically being the twin that will be born first (closest to the cervix). I am fairly certain that the techs switched which twin was A and which was B early on in my pregnancy, as there seemed to be initially a great deal of confusion. As they got bigger, it became very clear who was who, with my Twin A always head down and his brother floating around above him, frequently kicking him in the face and being caught on camera doing so!

One placenta or two?

Twin pregnancies are classified by number of placentas and amniotic sacs.

Di/Di Twins are dichorionic/diamniotic, which means there are two placentas and two amniotic sacs, one of each for each baby! My twins were like this in the womb, and now I also need one of each of everything in the playroom. Di/Di twins can be identical or fraternal (more on that later).

Mono/Di Twins share a placenta but each have their own amniotic sac. These twins are always identical.

Mono/Mono Twins share both their placenta AND their amniotic sac. These twins are always identical.

If a twin pregnancy is Mono/Di or Mono/Mono, the determination of “identical” is easy peasy. If a Di/Di pregnancy is a boy and a girl, the determination of “fraternal” is easy peasy.

But a Di/Di pregnancy with same-sex babies is tricky! Normally, when each baby has its own sac and its own placenta, it’s because each baby came from a separate sperm and egg which implanted into the uterus separately. However, if a single embryo splits BEFORE implanting in the womb, you get identical twins who, for the first time—and possibly last time for the next eighteen years—don’t have to share basically everything.

So are they or aren’t they?

In a nutshell: Same sex twins with their own placentas and amniotic sacs are usually fraternal (from two separate eggs/sperm) but sometimes they are identical (from a single egg/sperm combo that splits before implantation.)

Percentage-wise, same-sex, di/di twins are likely to be fraternal. About 70–80% of di/di twins are fraternal. So if we play it based on the odds, my twins were likely fraternal.

I will admit that I could not always tell my babies apart in the beginning. But babies all look the same! Also, SLEEP DEPRIVATION! Based purely on the odds, I was #teamfraternal. But there were other clues that left me with doubts.

For one, my older kids have dark brown eyes and dark brown hair and an olive complexion … just like me. (I am half Chinese, half Caucasian.) My husband (Caucasian) had lighter hair as a baby but has otherwise brown hair and dark greenish-brown eyes. If I hadn’t quite literally seen these pale, blonde babies come out of me, I am not certain I would’ve known for sure they were mine.

Although the odds of my di/di twins being fraternal seemed high on paper, in reality, I wondered what the odds were of having TWO separate eggs and TWO separate sperm meet up and produce BLONDE babies with fair skin and bluish-green eyes.

The alleles for blonde hair and blue eyes are recessive and the alleles for brown hair and brown eyes are dominant, and although there’s more to the genetics of hair and eye colour than just that, two dark-haired, dark-eyed parents are most likely to make dark-haired, dark-eyed babies.

There is blonde on my husband’s side: his father is blonde with blue eyes. And on my side, my paternal grandpa was the same. Obviously, we both carry these recessive genes, but that they would be expressed in two non-identical babies seemed unlikely.

Once you have twins, you start paying a lot more attention to twins in general, and you also start meeting a lot of sets of twins. I met a lot of fraternal twins who, unlike mine, looked far different than each other; some looked like they might just be cousins rather than twins. I also started seeing photos from other families with di/di twins who got confirmation that theirs were identical, and I would look at their twins and try to assess if they looked more or less similar to each other than mine.

How do you find out?

To find out for sure if fraternal twins of the same sex are identical, you need to do a twin zygosity test. This involves cheek swabs à la CSI from both babies that are mailed off to a private lab. We used International Biosciences, and the cost was just under $200. We waited until their second birthday to find out.

I decided it would be fun to do an “identical or fraternal reveal” for the occasion, so when the results came in via email, I forwarded them to my bestie without opening them. She’s really good at keeping secrets, so she opened the results, and ordered either two identical stuffed bunnies or two different stuffed bunnies for the boys to open up.

It was all very exciting!

Matching bunnies!

Side note: the most famous twins from my childhood, Mary Kate and Ashley, are apparently fraternal. But what I want to know is … DID THEY DO THE DNA TEST? Or are they just going off the percentages because they were Di/Di twins? It’s safe to say that mailing in cheek swabs was not a thing in the 80s.

But does it really matter?

Fundamentally, it changes absolutely nothing for us to know that our twins are identical. It doesn’t change how we parent them or the fact that my daughter can easily tell them apart but my son is often wrong, or simply says “one of the twins did it.” (And his sister will jump in to clarify which one.)

Well, it does make my life slightly easier in that I can now confidently tell people that yes, they are identical when they ask. (Because sometimes I would say yes, just to keep it easy, but then I’d feel like I was possibly lying, and other times I would say I didn’t know, which would lead to a lengthy explanation my interlocutor was not anticipating.)

I know this looks like a tender moment, but B is about to push A to the ground.

Personally, their faces look really different to me, and there’s a big enough size difference that if they’re standing next to each other, it’s easy to know who’s who. But based on my inability to tell other people’s twins (even fraternal ones) apart, I am never offended that others mix them up!

I am really curious if they will be mischievous enough as they grow up to try to trick people (teachers?!) into thinking one is the other. I’ve already been training my oldest to know that Twin A has an identifying mole on his leg if ever a flustered teacher is doubting who’s who.

I think it’s fascinating that my boys share the same DNA, meaning any of their respective children will be genetic half siblings. (Which again, makes really no fundamental difference, it’s just a cool bit of trivia.)

Their DNA might be the same, but their personalities are definitely different. I feel like there’s a lot more of their personalities to be discovered when they finally start talking. (We are doing speech-language therapy, but that’s for another blog post!)

Did you know that the identical/fraternal distinction isn’t so black and white?

Newborn photos courtesy of my pal and Calgary-based photographer, Evi Novak.

One response to “Fraternal or Identical? I Don’t Know!”

  1. Very interesting! I thought I knew the difference but I didnt realize di/di could be identical.

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My name is Lindsay and I am a 40-year-old mama of four trying to live an eco-friendly, budget-friendly life! I am a substitute teacher and Child Passenger Safety technician in Calgary, Alberta. Join me on my adventures!

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