Twins 101: Other weird and awesome stuff

This is part three in a series of indeterminate length. This part will not make much sense unless you either (a) know the difference between identical and fraternal twins and/or (b) have read the first two parts.

Part 1: How to tell identical twins apart from fraternal twins
Part 2: How to actually understand the difference between identical twins and fraternal twins

Other weird and awesome stuff: Or, why you might be a twin (sort of) and not know it.
Now, here’s where it gets really interesting. Remember when that first little clump of cells (scientists call it a “blastocyst”) split into two identical groups? That’s not the only thing that can happen!

(Note: The following illustrations are all greatly simplified. Obviously. This includes some of the terminology.)


In the very early stages—up to maybe the first two weeks after the egg meets the sperm—blastocysts can do all sorts of crazy things. Those two identical groups can recombine and form just one group of cells again—in that case, one baby will eventually be born, never knowing that he or she once had the potential to be twins! Or they can stay separate, but only one of them survives—the other either just disappears or is absorbed by the other or who-knows-what. (In very extreme* cases there are people who find that they have been carrying the skeletal remains of an unknown twin for their entire lives.)


Or… you can start out with fraternal blastocysts, and then each of them splits in two. This would put you in the line for quadruplets (assuming they all survive)… unless some of those blastocysts recombine.

You see, for that first period of maybe a couple of weeks**, one group of cells is perfectly happy either splitting in two or recombining… with any other group of cells. So, you could end up with a recombined blastocyst that has two different sets of DNA. If that blastocyst survives, it will eventually become a person who has a condition known as chimerism: one person with two genomes.


In many cases, there is no outward sign of chimerism. Most people are unaware that they have this condition until they are tested for some other reason later on in their lives.

Scientists currently think this is pretty rare, but nobody knows for sure. Why? Well, it isn’t something that is routinely tested! It’s quite possible that chimerism is much more common than we think! In the same way, it is quite possible that many—maybe even most—blastocysts go through the twinning process at some point. Some scientists have theorized that maybe everyone once had the potential to be an identical twin. Because we don’t have a good way to observe the conception process, there is really no way to tell what is “normal”.

We know what is the normal outcome (normally, in humans, only one baby is born at a time), but we don’t know how many blastocysts are routinely formed and then discarded or combined. (The answer might be: very few. Or it might be: a whole lot. We just don’t know.)

*And very creepy.^

**This is where the 12-14 period often used in embryonic stem cell research comes from. Before that time is up, the little group of cells has the potential to become one individual, two or more identical individuals, or even one individual with chimerism.^