When You Don’t Know if You’re Expecting Identical or Fraternal Twins

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Finding out you’re expecting twins is quite the surprize, and often followed by an explosion of questions. Whether you’re a first-time mom or a seasoned pro, twins are usually unchartered territory. Medical technology can tell us all kind of information about our unborn babies now, yet some of us find ourselves with unanswered questions. Why do some expectant parents know if their twins are identical and some don’t?

When you don't know if you're expecting identical or fraternal twins

My (Possibly) Identical Twin Pregnancy

I, like many moms, found out I was expecting twins at 8wks of pregnancy. At my first routine appointment, my OBGYN performed an ultrasound and announced there was not one, but two babies, how exciting! After I had picked my jaw up off the floor I asked “Are they identical?”, to which I was told it was too early to tell.

My next appointment at 12wks was a quick drop in appointment and the last with this particular OBGYN. I was about to move, from Texas to Arizona, and although an ultrasound was performed, it was a speedy look in and I was happy just to see two healthy heartbeats. “They’re probably fraternal and you’ll likely give birth around 36-37wks.” he casually said as he shook my hand and left. I was so shocked that I had just lost a week of pregnancy prep (I hadn’t even considered premature birth) that I didn’t ask anything else.

My identical boys, born at 36wks

Between a trip to Europe and moving house to Phoenix, it was another 7weeks before I got to see my babies again. “Your previous OBGYN has noted them as fraternal but I don’t see two placentas.” said the ultrasound technician. “Were the placentas looked at during your last ultrasound?” I explained that my last appointment had been a quick one and there was no specific measurements or conversation about the placenta.

A detailed inspection of the placenta was carried out, carefully looking for a tiny junction between two potential placentas. Maybe they had fused? Maybe they were just really squashed up together? Maybe my records were wrong and I was actually carrying identical twins sharing one placenta? I was informed there really was no way to be sure because they were both boys and I had missed the window to confirm one or two placentas. I would just have to wait and see!

identical or fraternal twins
No-one could tell them apart!

I was treated as if they were Mono/Di twins (one placenta, two sacs) because this was the higher risk situation. I’m glad I was carefully monitored because sure enough, when they arrived, two identical boys were born and only one placenta followed. What a wonderful surprise!

How to Know for Sure You’re Having Identical Twins:

  • During an ultrasound, a single gestation sac is clearly identified. (Mo/Mo twins).
  • During a first trimester ultrasound (usually 10-12wks), one placenta is clearly identified. (Mo/Do twins).

Although a single placenta and separate sacs (Mo/Di twins) are usually identical, the window of time to identify a single placenta is small. Around 40% of fraternal twin placentas fuse into one, usually around 13-20 weeks.

Different Types of twins expecting identical or fraternal twins

How to Know for Sure You’re Having Fraternal Twins:

  • During a second trimester ultrasound (usually 16-20wks), your babies are identified as different sexes.

Because I know someone will pull it out of the bag… there have been a handful of reported cases of genetic mutations in male identical twins, causing one of them to lose a Y chromosome and be born a female. Less than 10 cases of this have been confirmed. It’s extremely rare but Mother Nature can be wacky sometimes.

When You Don’t Know if You’re Having Identical or Fraternal Twins:

  • Two placentas are identified but the babies are the same sex. 25-30% of twins with two placentas (di/di twins) are in fact identical. The egg would have split very early on and allowed them to implant and grow separate placentas.
  • If you did not have an examination of the placenta(s) during your 12wks appointment and two gestational sacs are identified. As the placenta(s) grow it becomes difficult to know if you started with one or two.
It’s always been clear these two are identical from their looks!

How to Find Out if Your Twins are Identical after Birth:

  • When your babies are born the hospital should check their blood type. If they have different blood types, they are fraternal.
  • A twin zygosity DNA test. Similar to those ancestry DNA kits, you can send off some of your twins’ DNA in the mail and quickly find out if they are identical. Identical twins share 99.9% of their DNA, with natural mutations making up tiny differences.

Going back to those ancestry DNA kits… I recently met a man at the local children’s museum who was reminiscing as my boys were playing. He told me he was also an identical twin and we chatted for a short time. Turns out he had done a ’23 and Me’ DNA ancestry test but laughed when his twin brother’s kids did the same test and he was connected to them as the father… awkward, ha! The magic of matching DNA!

Whether you’re expecting boys, girls, identical or fraternal, I wish you all the best for a happy healthy pregnancy!

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About the author

Katherine is an electrochemist, hiking enthusiast, and family lifestyle blogger. As a mom of three, including twins, she enjoys DIY, travel, and eating good food. British born, Katherine moved to the US in 2014, and now called Las Vegas home.

3 thoughts on “When You Don’t Know if You’re Expecting Identical or Fraternal Twins”

  1. I’m pretty happy to live in a country where it’s routine to offer ultrasounds between 5-8 weeks. I scheduled my U/S appointment for 7 weeks, 6 days to give them enough time for a baby to be clearly visible on the screen.

    I got to see the screen when I first laid down. I remember looking at the screen and thinking that the gestational sac was bigger than anything I’d seen online. I was pretty ecstatic to see a baby in there and didn’t think much of it. She turned the screen away from me after she switched to the TV ultrasound wand. It felt like a lifetime between that point and being told that there were two babies. She showed me the screen. I knew they were identical because they looked to be in one sac. We found out later that it was two sacs and one placenta (mo/di).

    I think it would be a lot more difficult to tell with identical twins and two placentas. For me, it was super clear from the start. Great article!

  2. I too have identical twin boys! I knew from the beginning. I read everything I could find on the internet about them to try and prepare myself. We had a twin to twin transfusion scare, which luckily resolved itself in the nick of time. They were born at 37 weeks and are Mo/Di. In the beginning they looked exactly alike, of course I could always tell ?. But as they have aged (1.5 years now) they look less and less alike. I want to do a DNA test soon!

    • Oh how fun! So you think they may not be identical afterall? Maybe a fused placenta? It’s all so intriguing, love a bit of Twin science!


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