CHAPTER IV THE DURATION OF PREGNANCY
‘Tue duration of pregnancy is forty weeks or two hundred and eighty days, less the duration of an average monthly period, namely, five days. It is difficult to ascertain exactly the date on which impregnation occurs. For this reason the cal- culation is usually made by counting two hundred and eighty days from the beginning of the last menstrual period, assuming that impregnation takes place within a day or so after this period ends. It is supposed that labor comes on at what would correspond to the tenth monthly period. This is probably right, as labor, in all likelihood, begins from the same impulse as that which induces menstruation. A safe method of calcu- lating is to ascertain the date on which the last menstrual period began, count back three months, and add seven days. This gives the duration of pregnancy as nine calendar months, the seven days being added to allow for the completion of the men- strual period and the occurrence of impregnation succeeding it. This gives two hundred and seventy- three days as the actual duration. In the case of most women this is too short. But by accepting it as the average duration it gives a safe margin for the engagement of the nurse and the prepara- tion for labor. We must bear in mind that it is possible for impregnation to occur just before the approach of a monthly period, so that a period of approximately three weeks would elapse between the appearance of the last menstruation and conception. If this were the case the method of
29