Left Handed People, Listen Up

Most people are familiar with the two basic kinds of twins, fraternal (two eggs fertilized at once) and identical (one egg splits into two).  Actually.... I've also had a few discussions in the past few months that have introduced me to a third basic type.... the elusive "paternal twin", but I digress. 

Identical twins can also be split up into 3 different types, di-di, mo-di, and mo-mo which have to do with the number of placentas and amniotic sacs.  I've covered the details in previous blogs, feel free to dig back through the archives if you want a refresher.

There are, however, a few other types of twins which present some pretty interesting circumstances.  The first of which, while rare, is still pretty well known, conjoined twins.  These are sometimes referred to as Siamese Twins.  The term was coined from Eng and Chang Bunker, conjoined twins born in Siam who toured the world as a circus act.  An interesting story in of itself.

A second type of twinning happens later in the development of the egg, usually about 7-10 days after implantation, where the egg splits into a mirror image.  When this happens it results in mirror image identical twins.  With mirror image twins, they are identical in every way except they are mirror images of each other.  If one has a birth mark on the right cheek, the other will have a birth mark on the left cheek.  If one has a cowlick on the right of their head, the other will have one on the left, if one is right handed, the other will be left handed.   Mirror imaging is fairly common happening in about 25% of identical twins.

So why did I direct this post to left handed people?  It has long been assumed that left handedness is a genetic trait.  However, the frequency of left handed people remains pretty consistent across the globe, about 10%.  If it truly was a genetic trait, you would expect to find different frequencies of left handedness in different geographic areas and populations as you do with things like blonde hair, green eyes, and dark skin.  New research is starting to reveal that left handedness may be linked to something completely different.

With the rise in ultrasound use during pregnancy in the past 30 years or so, scientists have discovered an interesting phenomena called Vanishing Twin Syndrome.  Vanishing Twin Syndrome is exactly what it says it is.  A twin pregnancy is discovered via ultrasound early in the pregnancy and weeks or months later at a follow up ultrasound, there is only one, with no trace whatsoever of the other twin.  The body literally absorbs the other twin without the mother ever noticing a thing.

When first discovered it was believed that this was a rare occurrence, but new research says otherwise.

  Many experts now believe that 1 out of every 8 pregnancies begin as a twin pregnancy, even though only 1 in 70 pregnancies result in twins.

Also another interesting fact is that twins, regardless of which type, fraternal or identical (or paternal... I guess) are more than 2x's more likely to be left handed. 10% of the population is left handed, but more than 20% of twins are left handed

Left handed people... see where this is going?

Research is starting to conclude that left handed people have a significantly high chance of having started life as a twin.... and could very well have been a mirror image identical twin.  However, nature stepped in and decided that the world just wasn't ready for two of you.