It also pays to remember that whatever dating system is used, and however accurate or inaccurate it may be... all it can do is determine how long ago it was you probably conceived. And all that tells you is a very vague 4 week period over which MOST women will go into labor, which is the 4 weeks between week 38 and week 42.
The EDD of the very day of the 40 week mark, is nothing more nor less than, simply the 40 week mark, and calling it the "due date" tends to obfuscate the fact that, it's merely the middle point of the normal range of probability.
A certain percentage of women fall outside even that 4 week spread.
So, my EDD of January 13th, actually translates thus: "If I am like the majority of women, I will go into labor and birth this baby, any day between December 30th, and January 27th."
So much for "due" dates, yet we are encouraged to believe they are a deadline of sorts. They are the midpoint of an average that actually covers 4 whole weeks.