In the world of obstetrics, a woman's estimated due date is relied upon heavily by prenatal caregivers. However, the accepted method of calculation of the EDD is based on inaccurate data.
This article explains the flaws in the current method and how to more accurately estimate a baby's due date:
The Lie of The EDD: Why Your Due Date Isn't When You Think
"We have it ingrained in our heads throughout our entire adult lives-pregnancy is 40 weeks. The "due date" we are given at that first prenatal visit is based upon that 40 weeks, and we look forward to it with great anticipation. When we are still pregnant after that magical date, we call ourselves "overdue" and the days seem to drag on like years. The problem with this belief about the 40 week EDD is that it is not based in fact. It is one of many pregnancy and childbirth myths which has wormed its way into the standard of practice over the years-something that is still believed because "that's the way it's always been done".
"This theory was originated by Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the EDD based upon evidence in the Bible that human gestation lasts approximately 10 lunar months. The formula was publicized around 1812 by German obstetrician Franz Naegele and since has become the accepted norm for calculating the due date."
"Recent research offers a more accurate method of approximating gestational length. In 1990 Mittendorf et Al. undertook a study to calculate the average length of uncomplicated human pregnancy. They found that for first time mothers (nulliparas) pregnancy lasted an average of 288 days (41 weeks 1 day). For multiparas, mothers who had previously given birth, the average gestational length was 283 days or 40 weeks 3 days. To easily calculate this EDD formula, a nullipara would take the LMP, subtract 3 months, then add 15 days. Multiparas start with LMP, subtract 3 months and add 10 days. The best way to determine an accurate due date, no matter which method you use, is to chart your cycles so that you know what day you ovulate."
I've also read about the inaccuracy of estimated due dates in childbirth books such as: Your Best Birth, by Ricki Lake and Abby Epstein and The Thinking Woman's Guide to a Better Birth, by Henci Goer.