This is a great article revealing the flaws in the current system of due-date estimation, which are legion. Check it out!
"The Lie of the EDD: Why Your Due Date Isn't When You Think It Is"
Why are due dates usually so off? Many reasons, including the following:
(1) The current "length of pregnancy" allowed is generally too short, especially for first-time moms
(2) EDDs assume a 28-day cycle, which is not accurate for many (or most) women
(3) EDDs assume ovulation on day 14 (a 14-day luteal phase), which is not accurate for many women
For example, my own case... I have cycle lengths in the low-to-mid 30's (33-35 days) and a short luteal phase (10-11 days). This means that even using the modern system of due date estimation, my due date could be off by up to a week using LMP (last menstrual period) dating.
This issue is extremely important in modern obstetrical care. A dear friend of mine had a most-likely-needless cesarean because her doctor only "allowed" her patients to go to 41 weeks before induction. What followed was the usual package: induction, epidural from pitocin-induced contractions, failed induction and fetal distress, leading to an unnecessary cesarean. For her second, she quite understandably opted out of the whole mess and chose an elective repeat cesarean. Had her doctor understood the truth about due dates, the entire thing might never have happened, and my friend could have had two vaginal births.