Just a quick point to elaborate on twins being identical physically and the effect of environment.
This may or may not help the thread but it's interesting (to me at least) and as i recently had to answer an exam question on twins, it's still fresh in my mind.
Monozygotic twins (one egg, splits in two then implants in the womb and then grows) are rarely identical in size as measured during ultrasound examinations at about 20 weeks. It's only a concern when one is much larger than the other or some other measurement would point to things not being well.
Such a measure would be amniotic fluid volume (the stuff baby floats in - mostly composed of baby piss by the way). One cause of a shortage of this fluid is twin to twin transfusion syndrome (TTTS) - one twin shunts blood via aberrant blood vessels in the shared placenta to the other - meaning less blood and nourshment for one twin and therefore less pissing to be done . It's typically the twin shunting to the other that suffers and is smaller and slower to develop. The shunted to twin can often come badly too. In fact it can get so bad that the smaller twin can become "stuck" - kinda squashed up inside the womb if it has it's own sac to float in. This happens as one sac has way more fluid than the other.
Babies in utero can cope for a while if they are effectively starved in this way - an effect called brain sparing occurs where what nourishment there is goes to keep the brain growing and the rest of the body can swing for it. There are thankfully treatments for this.
There are many variations in the degree of TTTS and it can occur extremely rarely in non-identical or dizygotic twins too.
My point is (yes i promise there is one) is that environment does tend to affect us far earlier and in far more bizarre ways than we often think possible.
To those twins out there, carrying twins was risky for your mother, and risky for you too!
I personaly am amazed, since studying medical ultrasound, to learn about the whole pregnancy phenomenon, and how risky an event it is.
I for one have a new found respect for mothers for being able to pull of this feat, and for kids and especially twins for surviving it.
(Yes, I know that some folks here will go on about how the hell one can respect someone for doing or being something they have no control over - but even so, it's a risky business being a pregnant woman and being a fetus and that deserves something. Perhaps some ignorance is bliss after all........)