The evidence is in: differentiation in identical siblings begins before birth

The medical community has finally confirmed something most all parents of identical twins have always known: Our twins do not emerge from the womb purely identical. Differentiation begins when the egg splits and continues throughout their lifetimes.
Identical twins share the same DNA.
This new study does not dispute that.
Rather, the explanation for the differences lies in epigenetics, the way identical genes that accompany that shared DNA express themselves. Genes, scientists have learned, can be turned on and off like light switches. Those switches are flipped by environmental influences.
Epigenetics explains why identical twins can look somewhat different and have different inheritable diseases and conditions despite their common DNA.
I see it in my own guys.
At 5 years old, Matthew and Jonathan are precisely the same height. Their hands and feet are the same sizes and their hair grows in all the same directions. Yet, one is slighter than the other, overall. One has a higher-pitched voice. One has a spider vein on his face.
They have different tastes in food and different levels of immunity.
They are daring in different ways; shy in different ways.
They are very much identical, but they are different.
Before we had Jonathan and Matthew DNA tested, when they were still infants, so many people looked for those minute differences as proof that they were fraternal. Identical twins, especially infants, should be identical in every way, they believed.
It was annoying.
Some of those people were relentless.

Scientists had already proven such changes take place after birth, but researchers from Murdoch Children’s Research Institute (MCRI) in Melbourne are the first to prove the environment in utero can also be responsible for such changes.
The Australian scientists used cord blood, placentas and umbilical cords collected at birth from both identical and nonidentical twins to prove their theories, according to recent article in the Deccan Herald. They found that although identical twins share the same DNA sequence, the chemical markers that switch genes on and off are different.
That makes sense to me.
In that first ultrasound at 20 weeks, the day we learned I was carrying twins, Matthew was already set to escape. He had claimed the spot at the bottom of my uterus near the cervix, the head-down position usually reserved for a baby who is prepared to make an exit.
There he remained until delivery.
Jonathan seemed to spend the whole pregnancy trying to get comfortable. He was in breech position at delivery after flipping head-up and head-down a few times during the last weeks of pregnancy (That hurts, by the way!). When Matthew cleared out his space, Jonathan spent the next 20 minutes swimming, alluding the grip of my OB.
He became a c-section baby when he decided to take a pike dive -- head and foot first -- into the world.
Their experiences in utero were entirely different.
Why wouldn't that change them?
What does that mean for identical siblings? For parents of identical siblings?
It's hard to say.
The research team believes it might help track and treat diseases earlier in life. I'm not so sure about that. I can't imagine pediatricians will suddenly start testing the cord blood of all newborns for changes in chemical markers. It's not practical.
Their team leader also noted that it might help parents understand that certain elements of fetal development are out of the parents' control. That could be comforting for some. Disconcerting for others.

For me, personally, it's a scientific answer to questions I get all the time: how can Matthew and Jonathan have physical difference and still be considered identical? It's a scientific answer for all those parents who ask on online forums how their twins might possibly be identical when their birth weights are so drastically different or they are different heights.
It's a scientific answer to support what should be common sense.
No two people will ever be precisely alike.