How accurate is a typical due date for a pregnancy?  Specifically, where can
I find a fairly reliable histogram or graph of the probability
distribution of birth date as a function of predicted date of birth? 
I realize this might depend on the method of prediction and the date
on which it was performed.  I'm most interested in the data for
ultrasound during the first trimester, but I'd appreciate any graph
that gave me a real understanding of the variability of these dates.

Hi, 
Maybe some of the comments to similar question would interest you:

http://answers.google.com/answers/threadview?id=515106

How accurate?
Not very.

The standard deviation for normal human gestation period is about 15
days.  That means that about 1 in 3 babies will arrive either more
than 2 weeks "early", or 2 weeks "late".

Thanks for the information--it's consistent with what I'd heard.  It
is surprising that a more detailed distribution isn't available
anywhere on the web since, as someone who commented on the above
thread mentioned, it would be very easy for a hospital to gather this
data.