Zygosity: Do you know? Do you care?

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?

23 thoughts on “Zygosity: Do you know? Do you care?

  1. great post, I love this blog. I’m sharing on my FB as a lesson to my non-multiple friends :). I noticed the first bullet may have a typo, I think “way” should be “weight”. I hope you don’t mind me pointing out, I know I can never catch my own typos.

  2. my boys are monozygotic (they were monochorionic, triamniotic) but we didn’t find out until they were DNA tested after birth. don’t ask why my first OB didn’t know what “monozygotic” meant. whatever.

    i’m glad we know because a) it was the #1 question we got when the boys were born and b) i feel like it could make a difference with health issues. but mostly it’s just fun to know. :)

  3. We have no idea what my 15-month-old twin girls are, and I”m DYING to know! For those of you who have gotten your multiples tested, can you refer me to the cheapest / most trustworthy company?

  4. Great post! I’m always amazed by how much doctors and others don’t know about twins. We don’t know if our girls are identical yet and are researching companies to get them tested. Our girls were Dichorionic-Diamniotic but they look _a lot_ alike. Any recommendations on testing?

  5. Mine are monozygotic, and I’ll admit that it does matter to me. I would never have guessed that they were identical from looking at them, so seeing the single chorion was quite the breath-taking sight. I have a background in biology, so I knew what I was looking at.

    I think knowing that my girls have the same DNA, and at the same time seeing how different they are from one another, and how one takes after my husband and one after me, makes their twinhood that much more of a mystery to me. I think knowing that they’re monozygotic also makes me more forgiving of people who can’t tell them apart!

    I’m surprised by people who are surprised that my identicals are different heights, and a pound apart in weight at age 4. I suppose I just assume that people know that these things have a huge non-genetic component.

  6. Ooh, there’s some info here that’s different than what I read when mine were born in 2004! I like how the “facts” keep evolving. I’m excited about what else they’ll discover as my boys age!

    We don’t know whether ours are monozygotic. They were dichorionic/diamniotic, and we were told they were fraternal until after they were born. They have the same blood type, and look very similar.

  7. We were told fraternal from our 6 week US. Then we found out they were both girls and identical kept creeping in the back of my mind. The placenta testing came back inconclusive and they had the same blood type. As they got out of that fresh baby stage they had a bit of a different shape to their head but if they had hats on they were spot on. At 6 months we had them tested and it came back positive. I felt I needed to know because everyone asked and I didn’t feel like I could claim “identical twin” if we didn’t know for sure. There’s just something magical and mystical about identical twins and I wanted myself (and my girls) to know what kind of twin they are.

  8. Well, mine are definitely dizygotic as they’re fraternal B/G twins but I don’t think even if they were same sex, I’d care that much… only if if it meant something else like predisposition to certain diseases, etc.

    The interesting thing you pointed out was that language delays and learning disabilities seem to be more common among multiples – I will keep my eyes open for that :)

  9. I’m having a hard time understanding how “dizygotic twins may run in families and may be passed down by both men and women.” How do the men pass down the trait that influences how many eggs woman have during ovulation.

    The workshop that spoke about these twin facts, was it given by a researcher or educator? Sorry, I’m an engineer, if something doesn’t make sense to me I want to know what it was based on.

  10. Judy – men can pass down the trait to have dizygotic twins to their offspring, so if they have a daughter she may be more likely to have dizygotic twins, but if they have a son the son will still have that trait…just can’t have twins himself :)

    I don’t know if my twins are monozygotic or dizygotic. When I was pregnant I was told that they were fraternal because they had two sacs, but I have since learned that doesn’t matter. They look very, very similar and I suspect they are identical, but will wait sometime to find out. It really doesn’t matter to me, but people ask all the time and I have to say that I really don’t know.

  11. I think I understand after googling. The boy is XY, so can carry the X gene (with the reproductive traits from his mom) down to his daughter.. Sorry, was thinking the X stopped at the boy..Thanks for following up!

    Now about the post. I wonder if the cancer one applies to MZ and DZ multiples.

  12. Mine are identical. The ultrasound tech had the wand thing on my belly for all of 30 seconds and he could tell me 1. you have twins. 2. they’re identical. 3. This one’s a boy, so they’re both boys. Once I got over the shock of having twins, I realized what a marvel it is that the ultrasound tech could tell all that in literally 30 seconds (especially because even after all my ultrasounds, I can’t tell anything. I remember saying, “Is that the baby’s head?” and being told, “um, no that’s your kidney. We’re nowhere near the babies now.” LOL) It’s not important at all to know their zygosity, but like Pam said earlier, it’s fun and it’s the most asked question. Though even if my boys hadn’t shared a placenta and outer chorion, we’d know they were identical. Not only do they look almost exactly alike, but they’ve hit all their milestones on the exact same day, have the same rare blood type and are the exact same height/weight and have been since about 6 months.

    Regarding dad causing fraternal twins: It’s my understanding that a man can pass on the gene to ‘hyper-ovulate’ to his kids (thus affecting his grandchildren and great grandchildren being fraternal twins, and also contributing to the myth that “twins skip a generation”) but he can’t affect his wife’s ovulation, so your husband having twins in his family wouldn’t affect your chances of having twins. Does that sound right?

  13. After reading Nancy Segal’s book One and the Same I was fascinated to learn that twin researchers take it a step further and find it significant to know WHEN the egg split with monozygotic twins.

    If the egg splits before implantation (1-3 days past fertilization) they’ll have separate placentas. About a quarter of monozygotic twins have separate placentas and amniotic sacs. The majority split between days 4-8 and share a placenta but have separate amniotic sacs. 1-2% split after the 9th day and share a placenta and are in the same amniotic sac. If they split any later they are conjoined (very rare).

    Further confusing things it’s possible for two placentas to fuse and give the appearance of being one. I’m sure that there are many identical twins out there in the world that think they are fraternal because they had separate placentas and many fraternal twins that think they are identical because their placentas fused.

    It was at my boys’ 4 month visit to the pediatrician that the dr said that she thought they were starting to look more alike and that we should do the test if we were curious. They have the same (uncommon) blood type, A+, and I had a feeling all through my pregnancy that they were identical even though the odds were they weren’t. Turns out they are. Now they are 18 mo it’s becoming a lot more difficult to tell them apart.

    I’m glad we did the test. There are practical reasons for knowing if they are identical- we’ll be able to tell if one of them falls behind developmentally, we know they are cool with being on the same schedule (naps, feeding, etc) and tend to like the same things. It’s amazing how their teeth are coming in in the same order within days of each other, they started crawling within days of each other, and they even started walking 45 minutes apart!

  14. My twins girls are fraternal – night and day, really – blond/blue and brown/brown, completely different facial features, personalities, etc. and we still got questions from people if they were identical. Made me giggle every time. If they were more alike or if there was a question of identical/fraternal I think I would have found out to quench my curiosity but I don’t think it would matter to me either way. Interesting about the growth differences – my girls were 3 oz. different at birth and same height, now one is an inch & 1/2 and 1 lb bigger than the other.

  15. I was told my girl were identical at my 6 week u/s. I used to be surprised by how many people weren’t sure, but then I realized not everyone gets an u/s at 6 weeks!

    My girls are the epitome of the whole identical but look nothing alike scenario. Caden had placenta issues and so is much smaller. She also looks more like me, while Delaney looks like the sperm donor. No idea how that happened.

    My biggest beef with identical twin facts is when the mom claims the embryo split at 6 or 8 weeks. Just because you didn’t see two at the first u/s, doesn’t mean they split late! If they’re not conjoined, believe me, they split much earlier than 6 weeks!

  16. My boys are fraternal, one looks like my side of the family (and has hubby’s blood type), and the other looks like my husband’s side (and has my blood type) .
    At my first ultrasound (at 13 wks) they were in two distinct sacs, and even though the docs said there was a small chance they were identical, I was pretty sure they were fraternal. I have two biological sisters, and both have been pregnant with twins, on their second pregnancy (as was I). But, we’ve yet to find ANY ancestors with twins.

  17. My GBG triplets are fraternal. Not knowing if perhaps one had split very early in the pregnancy, we weren’t positive until they were born. They look as alike as any singleton siblings, but will never be mistaken for identical. They are blond/blue, brown/brown, and blond/brown. I hate that at 14 months old there are still people who can’t (or won’t) remember which of my girls is which (they’re the blond/blue and brown/brown). Everyone learned our nieces names correctly and right away, but because ours came all together some relatives still couldn’t remember at one year old.

  18. I have 11 mos old twin boys, and we just ordered a DNA test for them. They look very similar (most of the time- some days more so than others!) but have very different personalities. They got their teeth in the same order, though slightly different timing. They have the same blood type and have always been within 1/2 an inch and 5 oz of eachother.

    We had decided to wait on the DNA test to see if they looked very different as they got older. We will open the results on their 1st birthday as a celebration! I used Affiliated Genetics, and the cost was $160.

  19. Thanks Janna. I agree, the husband has no factor on if his wife will have twins, but it will factor on their daughter will have twins.

    About the fused placentas. I had a u/s at 10 weeks and the technician did throw that idea about there, except that she said that the “wall” looked to be too thin to be a fused placenta. the wall was extremely thin compared to the placental wall. I don’t know if this matter, but the girls were oriented in the exact same way in the u/s too. It looked like the top one was in a hammock. Yes, hammock instead of bunk beds..the seperation line was very thin.

    Yes, I think knowing if they are identical does matter because knowing that if one hits a milestone you can plan on the other one hitting it too. Teeth come in a the same time, so be prepared to hear some fussing, etc..It also makes the whole thing more interesting when you think about how nature vs environment affects their personalities, etc.

  20. My instincts have always been that my girls are dizygotic. They were in separate sacs with separate placentas and I have a long history of fraternal twins on both sides of my family (including my mother and father directly).

    Still, my girls do look very similar. They got their teeth in the same freaky order (top ones first) within days of one another, crawled on the same day, and have always been the same size. They are doing things so much like each other but very unlike their older sister that it has made me wonder. Mostly though, I just chalk it up to being twins and I have no desire to confirm that they are dizygotic.

  21. Hi, my twin girls will be 1 in Dec and we are thinking of getting a DNA test to celebrate. The debate has been going on since I was pregnant as the girls had two sacs and placentas so could be either. They look alike but one is just bigger. My sisters are identical so it would be interesting to know if my two girls are too.

  22. That is so interesting to hear that it can pass down from the father’s side. There are 2 sets of twins on my mom’s side but was told my a doc that mine can’t be genetic b/c my mom should have been the one to have twins (my grandmother was a twin). Now it makes more sense. There are 3 sets of twins on my dad’s side. So…with 5 sets of twins surrounding me I didn’t have a shot, did I? I wouldn’t have it any other way. I love having twins…now that they are 6!!! Wasn’t easy for the first 3 or 4 years tho.

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