I think of Frodo telling Gandalf: "You're late." And of course Gandalf says: "A wizard is never late, Frodo Baggins. Nor is he early. He arrives precisely when he means to."
Let's ask America's midwife Ina Mae Gaskin what she thinks.
"Over the last decade or so, many women have come to believe (erroneously) that it is safe -- even good -- to have their labors induced by the time they get to 38 or 39 weeks of pregnancy. As they approach their estimated due dates, many women undergo a lot of pressure from well-meaning friends and relatives to choose labor induction. One of the unfortunate results of this social/medical fad is the birth of the most preventable category of premature infants, those whose due dates were misestimated. This is a common mistake, since many women don't remember the date of their last period and ultrasonographic diagnoses aren't always accurate.I'm reading an eighth edition 1948 Handbook of Obstetrics. While this was a time of new ideas for regulating birth in the hospital, obstetrics still recognized wide variation in pregnancy.
Contrary to many people's beliefs, a pregnancy is not post-term until 41.5 weeks. My practice experience indicates that a 42 or 43-week pregnancy may be optimal in some women. My conclusion is that it doesn't necessarily make sense to induce labor for that indication alone." (317-318)
"The length of pregnancy varies greatly; it may range, indeed, between such wide extremes as 240 days and 300 days [34-42 weeks], The average duration, counting from the time of conception, is nine and a half lunar months, that is, 38 weeks, or 266 days. Counting from the first day of the last menstrual period, its average length is ten lunar months, or 40 weeks or 280 days. That these average figures mean very little, however, is shown by the following facts. Scarcely one pregnancy in ten terminates exactly 280 days after the beginning of the last period. Less than one-half terminate within one week of this 280th day. In 10 per cent of cases birth occurs a week or more before the theoretical end of pregnancy and in another 10 per cent it takes place more than two weeks later than we would expect from the average figures cited above. Indeed, it would appear that some children require a longer time, others a shorter time, in the uterus for full development. In view of the wide variation in the length of pregnancy, it is obviously impossible to predict the expected day of confinement with any degree of precision."It goes on to explain how to count from the last period and notes:
"While it may be satisfying to the curiosity to have this date in mind, it must be understood that the likelihood of labor's occurring even withing a week of this day is less than 50 per cent. There is one chance in ten that it will come at least two weeks later. And yet, whether pregnancy terminates a week before or two weeks later than the day calculated, the outlook for mother and baby is usually just as good as if it had ended at 'high noon' on the due date. Actually, women seldom go 'over-term'; in most cases it is the above system of calculation and not Nature which has erred. For example, ovulation and hence conception may have occurred some days later than calculated; this error would make the beginning and the end of pregnancy just that many days later. If, superimposed on this circumstance, we were dealing with a baby which required a slightly longer stay in the uterus for complete development, it would be clear that the apparent delay was quite normal and for the best." (88-89)What do you say when a friend asks, "When is he due?" I like to say, "He's a spring baby." Or "My guess date is December 20th." This time, 40 weeks happens to be in the middle of the month, so I say, "He is due in August." If my friend wants to know what day exactly, I explain, "He is full-term anytime during the month." Sometimes I grin, "I don't know - he hasn't told me yet."
More info? Try Spiritual Midwifery by Ina May Gaskin.