I love statistics! and I too was curious about this. this is the best I could do with 10 mins of surfing:

http://en.wikipedia.org/wiki/Pregnancy#Duration

^ Dr Sally Tracy, Having a Great Birth in Australia ed. David Vernon, Australian College of Midwives, 2005, p22

Fewer than 5% of births occur on the due date; 50% of births are within a week of the due date, and almost 90% within two weeks.[16] It is much more useful to consider therefore a range of due dates, rather than one specific day, with some online due date calculators providing this information.
I think with some more surfing and changing search terms etc one could find the answer to this. Some of this scientific stuff can't even really be found on the internet, it's in jouranls and whatnot, so if you're really keen on it you can look in a book or a journal at the library or university library near you.