SUGGESTIONS FOR CALCULATING THE DATE OF ANTICIPATED BIRTH
The leading authority in the _Manual of Human Embryology_, edited by Franz Keibel and Franklin P. Mall in two volumes, London, 1910, says:--
"In ancient times it was generally believed that the duration of pregnancy in man, unlike that in lower animals, was of very uncertain length; and it was not until the seventeenth century that it was more accurately fixed, by Fidele of Palermo, at forty weeks, counting from the last menstrual period. In the next century Haller found that if pregnancy is reckoned from the time of a fruitful copulation it is usually thirty-nine weeks, and rarely forty weeks in duration. In general these results are fully confirmed by the thousands of careful data collected during the nineteenth century."
"However, from thousands of records it is found that the mean duration of a pregnancy varies in first and second pregnancies, is more protracted in healthy women, in married women, in winter, and in the upper classes."
"From these figures it is seen that most pregnancies take place during the first week after menstruation, and that the duration of pregnancy is longer if copulation takes place towards the end of the intermenstrual period. And this is explained if we assume that in the first week, especially the first few days after the cessation of menstruation, the ovum is in the upper end of the tube awaiting the sperm and that conception immediately follows copulation. When the fruitful copulation takes place in the latter two weeks of the month the opposite is usually the case; the sperm wanders to the ovary and there awaits the ovum; and, therefore, on an average, pregnancy is prolonged in this group of cases, when determined from the time of copulation."
"In determining the age of human embryos it is probably more nearly correct to count from the _end_ of the last period, for all evidence points to that time as the most probable at which pregnancy takes place."
On the whole it is generally found that 280 days (_i.e._, 40 weeks) can be reckoned as the average period during which the child develops internally if the date is counted from the first day of the last menstrual period and 269 days if estimated from the date of actual union.