This article blew my mind. Warning: Math ahead.
"So what’s the probability of your existing? It’s the probability of 2 million people getting together – about the population of San Diego – each to play a game of dice with trillion-sided dice. They each roll the dice – and they all come up the exact same number – say, 550,343,279,001."
http://blogs.law.harvard.edu/abinazir/2011/06/15/what-are-chances-you-would-be-born/