Something I wrote last year...enjoy.
Due dates are a bunch of nonsense. Let me explain why:
1. Only 5% of women go into labor on their due date (In other words, it is wrong for 95%!).
2. The notion that pregnancy lasts 40 weeks from the last menstrual period came from a German physician in 1807 (Hermann Boerhaave). He didn't do any statistical studies on average length of pregnancies--he just came up with this number based on observations. In part, because it was a nice, easy number to remember (10 lunar months). This method of calculating due dates later became called "Naegele's Rule" and is the one that most doctors use today, even though there is no statistical evidence that 40 weeks is the correct average length of pregnancy.
3. Naegele's Rule is based on the last menstrual period and assumes a 28-day cycle. However, many women have cycles longer or shorter than 28 days. Ovulation usually occurs 14 days before the next period (NOT 14 days after the previous), so a woman with a 35-day cycle will ovulate later in her cycle than a woman with a 28-day cycle.
Let me illustrate this with an example: Let's say there are 3 women whose menstrual periods all began on the same day. I'll say Jan 1st for convenience.. They all became pregnant during the same cycle. Naegele's Rule (and most doctors) would give them all the same due date. However, that is flawed:
Amy has regular 28-day cycles. Her 40-week "due date" would fall on Oct 7.
Becky has regular 35-day cycles. Her 40-week mark is Oct 14.
Cindy has irregular cycles, ranging from 30 days to 55 days. Thus, her 40-week mark could be on Oct 9, or Nov 2, or anywhere in between.
4. The average length of pregnancy is not 40 weeks (okay, technically 38 because there are 2 extra weeks thrown in there before conception, but that is confusing to most people so I'll stick with 40). Women gestate babies for different amounts of time. Just like some of us grow faster, hit puberty at different times, or start our periods at different ages, women also grow babies at different rates. For example, in studies done of healthy Caucasian women, the average length of pregnancy was 41 weeks 1 day for first babies, and 40 weeks 3 days for second babies.
Next, you'll remember from statistics class that averages are calculated from a wide range of possibilities. Term pregnancy is generally considered to be from about 37 weeks and beyond. Many women go into labor between 37-42 weeks. However, a substantial minority gestate much longer--often 44 or 46 weeks. The longest documented pregnancy is 52 weeks. Yes, one full year. And you thought "Ten Month Mamas" had it hard!
So, even if you take into account the length of your menstrual cycle, and the fact that average pregnancies are 41 weeks 1 day for first babies if you're Caucasian, that still tells you nothing about what specific day you will go into labor. Hence the 95% inaccuracy rate. It just gives you a 5-6 week (or more) window in which your baby will probably be born.
5. Ultrasound dating is not 100% accurate, and gets worse farther along in pregnancy. 1st trimester estimates are +/- 5 days, while 3rd trimester estimates are +/- 22 days (or a 44-day window of possibility).
6. Way too many women go through unnecessary stress worrying about their due date. No wonder, since 95% of them are wrong!
I got most of my information from Anne Frye's Holistic Midwifery Vol 1, and Henci Goer's The Thinking Woman's Guide to a Better Birth.