How Are Twins Formed?

If you're pregnant with twins it might come in handy to know how twins are formed. This way you'll know whether your twin babies are fraternal or identical.

By Kate Phillipa Clark

How are twins formed? It depends on whether your twins are identical or fraternal and whether or not they share a placenta.

Identical twins

This type of twin is formed from one fertilized egg being split in two. The eggs develop into two individual embryos that share almost the same genetic material. When developing in the womb, identical twins can share or have their own individual placentas. The same goes with membranes. This sometimes makes it tricky to determine whether or not twin babies are identical or fraternal. Read more about how identical twins are formed to learn how to distinguish identical and fraternal twins from each other, or use our free Twin Test.

Fraternal twins

This type of twin is formed from two eggs being fertilized by two different sperm cells. Fraternal twins are genetically speaking like siblings. They may feel a bit closer, though, as they share a womb and are born at about the same time. As with identical twins there are some patterns of fraternal twinning that are exceedingly rare – read about the different types of twins.

How are twins formed? We hope we’ve answered your questions. Otherwise post a reply below this article and we’ll do our best to assist you. 

About the Author

Kate Phillipa Clark

Kate Phillipa Clark has a bachelor in Journalism and an Executive Master in Corporate Communication. She is an identical twin and so is her father.


  1. kjh / 19. January 2019 at 4:41 am /Reply

    I have fraternal twin girls and they are like night and day. One is tall, the other is short. One like to eat just about everything, the other is EXTREMELY picky. One is very studious, the other is athletic. As a matter of fact, they both told me that for their birthday this year, which is on Valentine’s Day, one wants a chocolate cake and wants to go to the fun station-the other wants a buttercream cake and go ice skating. The only thing that they could agree on was that they wanted separate days to celebrate their birthday. One decided to take February the 9th and the other took February the 16th. But neither of those dates are even their birthday!!!LOL!!!

    • Kate Phillipa Clark / 21. January 2019 at 8:18 pm /Reply

      LOL! No copy-paste routine at your house – Sounds like you’ve got two wonderful (but very different) daughters!

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