In a comment on the FutureBaby betting thread, Rivka writes:
(I’m laughing at all these people who are predicting early. The average gestation for a first time mother is 41 weeks, 1 day.)
Of course, as a physical scientist, my immediate reaction to that is “If they know that, then shouldn’t that be figured into the projected delivery date?” It’s not, though– we can count up the weeks, and the projected date is at exactly forty weeks.
So why doesn’t the projected delivery date reflect the average gestation time for a first-time mother? As someone who has taught pre-med physics, I have an extremely cynical and unflattering guess, but I’d love to hear an actual explanation.