Pervasive Misunderstandings About the Nature of Identical Twins

When my husband and I learned we were having twins, our first question was, "Are they identical or fraternal?"  I used to think, like most people, that this was an easy question to answer via early ultrasound, that identical twins shared a containing membrane (ie. they were in the same sac) and fraternal twins did not.  So all you would need to do was decide whether the babies shared a sac or not to know if they were identical or fraternal.  But it's actually more complicated than that...

Just to review the basics, identical twins are formed when one egg is fertilized by one sperm, which then splits in two.  The twin embryos formed are consequently genetically identical to one another meaning that they will have the same sex and will look very similar.  The reason I say "very similar" instead of "identical" is that even identical twins have some small differences in appearance.  Any of us that have known a pair of identical twins know that their mothers, at least, can tell them apart by looking, as can, in most cases, their friends and family.  And their personalities and individual talents are often wildly variable.  How do we account for these
differences?  Well, while the two individuals may have been built from the same blueprints, subtle differences in their environment, small variations in the way those blueprints are followed, little "mistakes" in their formation make them different.  By the time identical twins are born, they have been growing separate from, if very near to, each other, for 9 months, and are already slightly different.


Fraternal twins, on the other hand, are formed when two separate eggs are fertilized by two separate sperm.  They have nothing more in common, genetically, than do siblings of shared parents born at different times.  Therefore, while identical twins will always be the same sex and look "very similar", fraternal twins will just as likely have different sexes as the same, and have no more chance of looking alike than any other siblings.

So now that we are clear on what an identical vs fraternal twin is, how do we tell from an ultrasound which we are looking at?  The part of the general understanding that is correct is that twins sharing an amniotic sac are identical.  However, while all twins that share an amniotic sac are identical, not all twins that have separate sacs are fraternal.  Identical twins can present one of four ways, actually, depending on when the split occurs. If the split occurs very early, before the embryo begins to implant in the uterus, they will implant separately and will each have their own placenta and their own amniotic sac (diamniotic, dichorionic twins).  If the split occurs a little later, they will each have their own sac, but may share the placenta (diamniotic, monochorionic).  If the split occurs still later, after both the sac and the placenta have begun to form, they will share both the amniotic sac and placenta (monoamniotic, monochorionic).  And if the split occurs very late, they will be conjoined (aka Siamese twins).  The first is actually the best possibility, from a medical standpoint, because sharing placentas, and particularly sharing both placenta and amniotic sac (often abbreviated MoMo twins, for monoamniotic and monochorionic), can lead to significant problems in pregnancy for developing twins, due to unequal sharing of their blood supply (twin-to-twin transfusion syndrome).

So, if on ultrasound, you can tell that the twins share a sac and/or share a placenta, then you know they are identical.  However, if they are diamniotic, dichorionic, meaning they share neither a sac nor a placenta, you can't know for sure whether they are fraternal twins or identical twins that split very early (although fraternal is statistically more likely).  The only way to know for sure that twins are fraternal before they are born is if both genders are identified and they turn out to be different, as is the case with our twins (ie. we are having a girl and a boy).  If they share a sex, it can't be known whether they are identical until they are born.  And even then, if they are similar looking, the only way to know for sure whether they are identical or fraternal is to run DNA tests on them.

I only learned all of this when I began researching the topic in reference to my own twins.  It is interesting how many people, including twins and parents of twins, believe that two sacs always means fraternal.  I have met other pregnant women online who assure me that they are having fraternal twin boys, or fraternal twin girls.  But they can't know for sure!  And I am particularly suspicious now that there are many identical twins in the world who are under the false impression that they are fraternal twins who merely happen to look particularly alike.  I have known two sets of supposedly fraternal twins, myself, who were so similar in appearance that I would have trouble telling them apart upon first meeting them.  My guess is that these twins' mothers were told, incorrectly, that they must be fraternal, and that they never questioned it further.  After all, siblings can look quite similar anyway.  But without a DNA test, which I'd be willing to bet they never ran, they can't know for sure.  I am particularly curious to know whether the famous Olsen twins, who claim to be fraternal, yet who are similar enough in appearance to have played the same character on a television show, have ever run a DNA test to confirm that they are, in fact, fraternal.

So, if any of you reading this are a fraternal twin who looks strikingly similar to your brother or sister, maybe it's time to get tested.  After all, wouldn't it be nice to know that you've got a perfect match if you're ever in need of bone marrow, a kidney, a piece of liver or a lung?