How Are Conjoined Twins Formed?

by | Jan 3, 2008 08:00 PM ET

Welcome to Brain Stuff from where smart happens.

Marshall Brain

Hi, I'm Marshall Brain. With conjoined or Siamese twins in the news every day now, one question that comes up is how do conjoined twins form? There are two kinds of twins: fraternal twins and identical twins. Fraternal twins happen when the mother releases two eggs and they both get fertilized. Identical twins happen when a single egg splits after it's fertilized. Usually, this happens four or five days after conception when the embryo is made up of 30 cells or so.

The cells form into two separate clumps and the clumps grow into two identical babies. If the split happens later say at ten days old, you get two babies sharing the same sac inside the placenta. If the split happens even later say at 14 days old, the two clumps of cells may not completely separate from one another. This is where conjoined twins come from. If you'd like to learn more about this and thousands of other topics, come to


For more on this and thousands of other topics, visit and don't forget to check out the Brain Stuff blog on the homepage. You can also follow Brain Stuff on Facebook or Twitter at brainstuffhsw. The iPhone app is coming soon. Get access to our content in a new way. Articles, videos and more: all on the go. Check out the latest podcasts and blog posts and see what we're saying on Facebook and Twitter. Coming soon to iTunes

More To Explore