"The length of "term" pregnancy varies greatly; it may range, indeed, between such wide extremes as 240 days and 300 days, and yet be entirely normal in every respect. The average duration, counting from the time of conception, is nine and a half lunar months; that is thirty-eight weeks (266 days). Counting from the first day of the last menstrual period its average length is ten lunar months (forty weeks; 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 percent of cases birth occurs a week or more before the theoretical end of pregnancy, and in another 10 percent it takes place more than two weeks later than we would expect from the average figures cited above. Indeed, it would appear that for full development some children require a longer time in the uterus, others a shorter time.
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. The time-honored method, based on the above average figures, is simple. Count back three calendar months from the first day of the last menstrual period and add seven days.... 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 within a week of this day is less than 50 percent. 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 child 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 of these 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 usual; this would throw both beginning and the end of pregnancy just that many days later. if, superimposed on this circumstance, we were dealing with a child who 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."
