The Science of Twins

Twins. There’s so much to know. But let’s talk the science of it all.

There are 4 common types of twins:

  • Dichorionic/Dichorionic or di/di
  • Monochoronic/Dichorionic or mono/di
  • Monochoronic/Monochoronic or mono/mono
  • Conjoined Twins

There are other types, like mirror image twins, etc. But for this post, I need you to know these 4 specifically.

I’m also going to write this in simple terms. You won’t see me using zygote or fetus. We’ll keep this simple… it’s a baby.

In The Womb

There are two different sacs that are present with every pregnancy.

The outer sac is called the chorion, while the inner sac is called the amniotic. Most people are familiar with the amniotic sac. This is where the baby actually resides.

Di/Di Twins

Di/Di Twins
Di/di twins will have their own chorion and amniotic sac. They will also have their own placenta.

Di/di twins, that come from the same egg and sperm, split between day 1-3.

About 75% of the time, di/di twins will be fraternal. But fraternal twins also occur when two eggs meet two sperm, and two babies are created. It’s possible that di/di twins are identical but it’s not common.

Boy/girl twins are always di/di twins and are not identical. That may sound obvious, but I have heard from many Moms of boy/girl twins being asked if they’re identical. When one twin has a penis, and the other a vagina, they are not identical. Just sayin’.

Mono/Di Twins

Mono/Di Twins
Mono/di twins come from the same egg and sperm, and split between day 4-8.

Mono/di twins are identical about 95% of the time.

Mono/di twins will share a chorion sac, but have their own amniotic sac. They also share a placenta.

There are cases, where di/di twins become classified as mono/di twins because the two placentas will fuse together. Thus, when being viewed on ultrasound, it looks like just one placenta.

Mono/Mono Twins

Mono/Mono Twins
Mono/mono twins are very high risk. They are always identical. The egg splits between days 8 to 13. Not only do they share the chorion sac, but the amniotic sac, and placenta as well. They can physically touch each other. Because of this, there is a very high risk of their umbilical cords becoming tangled.

Conjoined Twins

Conjoined Twins
Conjoined twins split after day 13. They are mono/mono twins, who didn’t complete the split process and are thus conjoined at one body part. They are the most high risk; and will share everything: chorion sac, amniotic sac, and placenta.

Henry & Oliver

Our boys are mono/di. In the womb they had their own amniotic sac, shared a placenta, and are considered identical twins.

Twins Carried Full Term

During my pregnancy, I joined a lot of pregnancy forums that I am still active and participate in. I have a lot of interaction with Moms of Multiples and have yet to find a woman who’s doctor allowed her to go past 38 weeks of pregnancy.

33 Weeks Pregnant with Twins

33 Weeks Pregnant with Henry & Oliver

What I have read, a lot, is that twins develop faster in the womb. Their lungs are stronger, and they learn the process of suck, swallow, breath faster. I got to see my own version of this being truth with all my NICU babies — Zadey and the boys spent time in the NICU.

So, full term for twins is considered 36 weeks. There have been studies that find with twins, the placenta breaks down faster and after 38 weeks there is a higher chance of stillbirth. Because of this, most twins are delivered between 36 to 38 weeks. However, mono/mono twins are delivered earlier due to their higher risk and cord entanglement.

DNA Testing

There is only one thing you can do to determine if your identical twins are indeed, identical. It’s DNA testing. The test costs about $100 and the results come back fairly quickly.

The hospital where I delivered the boys is a teaching hospital; and they offered this service to me for free. They also did a complete examination of my placenta. Weird.

My last day at the hospital, I was informed that the boys are identical. I am not sure how they came to this conclusion completely, and I want to get my own DNA test done on them because I’m fairly confident that they are fraternal. I also have many bets stating that they are.

Twins & Family History

Identical twins are spontaneous. Genetics does not play a roll. It just happens.

Fraternal twins are not spontaneous, and genetics do play a roll.

Keep this in mind when you’re asking every twin Mom if “twins run in the family.”

Henry & Oliver are identical. This means it just happened. I could have 100 sets of twins in my family, it won’t make me any more likely to have twins.

The family history of twins for their Sperm Donor, is irrelevant. He could also have 100 sets of identical and fraternal twins in his family, and it would make no difference. Men get zero credit for creating twins.

A woman’s body will either split an egg, or release two eggs. Neither can be credited to the Sperm Donor.

But, I’m sure you want to know. There’s only 1 set of twins in my family, that I know of. They are identical girls, and my cousins from my Aunt Dale and Uncle Rick. This is my Dad’s side of the family.


  1. Bobbie says

    Interesting! I had no idea there was so many different types of twins. I mean I never looked into the logic of it though. I thought they were either identical or not. Having said that, some of these terms do come to surface when I think back to my twins situation. I actually had to dig the papers out and look. Mine were di/di. At the time I didn’t pay attention to the technical terms. I just wanted to make sure the one surfing was still going to survive :) interesting.. I should read more about it though :)

    • says

      I had assumed that yours were di/di. Sadly I came to this conclusion because you lost one and carried the other to term. That’s a very rare case for that to happen in anything but di/di twins.

  2. says

    Nice explanation Katy and kept as simple as possible. I was looking for a simple explanation for the events that lead to conjoint twinning, when I bumped into your article. I am about to do a short and simple story on Knatalye and Adeline aka Hope and Faith; the 10-month old conjoined twins just separated in Texas children’s Hospital. Will adopt your image and reference your blog. Thanks.


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>