The short answer is yes. Because that is exactly what has happened to us. All through my pregnancy we were told my girls were fraternal, also known as Dizygotic or Non-identicals. They had their own separate houses (sacs) and their own separate lunch boxes (placentas). So how did all my medical examiners get it so wrong? Apparently quite easily! First let’s have a quick look at how twins work!
Fraternals are made when two separate fertilised eggs are implanted into the uterus wall. Two eggs were fertilised by two different sperm cells and so created two separate babies during the same fertile period. They have their own houses and lunch boxes and are the usually the safest twins to carry as they are not sharing food sources or space. They are also the most common twins. As they are not identical half of all fraternals are made up of male-female pairings, with the other 50% equally split in chance of male-male or female-female outcomes.
The chances of these twins increase with age, as the body produces more eggs each ovulation as you grow older. IVF is also another way women regularly fall pregnant with non-identical twins, as more than one egg is implanted at a time.
Dizygotic twins are essentially two separate siblings who have been carried at the same time. So count yourself lucky (if you can call it that!) that you only have to go through pregnancy once but get two beautiful siblings at the same time!
Identical twins, or Monozygotic, are made by one egg being fertilised. This egg than splits into two different embryos. Identical twins only make up around 3 of every 1000 births in the world. So they’re pretty rare compared to your normal average Joe baby.
Up to 70% of mono twins share a placenta but have separate sacs. Up to 30% have completely separate womb lives (such as my girls did) with only 1% of mono twins having to share their sources of food and living space entirely with their twin.
Identicals are always the same sex, unless a mutation occurred in the womb but are never truly 100% identical. Fingerprints are a great example. As the twins touch different parts of their surroundings during development, each fingerprint will have tiny differentiations to their siblings. Allergies are also another way they are different. A set of twins my partner knows sees one twin with a case of asthma, while the other twin is completely asthma free. My girls are another case. Beau has CMPA but Bebe is totally fine with cow’s milk protein.
Cool fact about identical twins – siblings of identical twins will genetically test as half-siblings rather than cousins. Some factors affect this but in general this is what will scientifically happen. I think that’s quite interesting anyway haha!
The chances of twins occurring at all are really only affected by the mother’s side. Family history, her egg production etc. With the male parent having no real effect on whether or not a twin occurs. Other types of twins can occur, including the famed conjoined twins. Have more of a read on other types of twins that can occur here.
So how did my girls get confused?
It all depends on your twin’s degree of separation. In other words when did they split into two embryos, if at all?
This image (source) shows what type of twins you will get from each stage of separation (cleavage). This cleavage date determines the sacs and placentas.
It is a common misconception that separate houses and food supplies mean fraternal twins. If mono twins split early enough in development, the arrangement of sacs and placentas make it almost indistinguishable from dizygotic twins. As dizygotic twins are much more common, it was only natural for the medical professionals to assume that that was what my girls were.
Although it would have been nice, if at some point, someone had told me that it was still a possibility that they were identical. Only during the end of the pregnancy and after they were born did I read/hear that they might still be mono twins. A lady stopped me in the hospital a few weeks after the girls arrived and explained about how her twins were identical and they only knew when they grew up a little bit as they were always told they were fraternal. She explained to me about the egg splitting before anyone saw and from that point I watched every growth the girls made.
I studied their noses and their eyes. I looked at their lips and their little ears. As a whole they looked different because Bebe was so much smaller. But each feature was identical. After many people had asked me if they were identical I finally caved. I googled online and found a DNA company who specifically did a test for twin profiles to see if they were fraternals or identicals.
We sent off the DNA and less than a week later we got a certificate confirming that our girls are identical! Although the girls were special before, now they are just super special when I know that they are kind of half of the same person. But I have learnt already that being half of the same person does not mean that they are the same person. They each still have their own traits and personalities (and allergies).
So that’s how my twins were mixed up as fraternals but were identical. So if you have been told your twins are fraternal but you have suspicions, you might be right! You can also get them tested at the point of birth if you save the placenta. Another, free, way you can tell is to have patience. As they get older their hair and eye colour will be identical. Their faces. Their features. If you have patience. Which I clearly do not!
The DNA company we used can be found here. The test was easy and from start to finish the whole process was done quickly (a lot quicker than their given expected time frames).