“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. Leuckart tabulated results from a large number of births which took place within the first ten months of marriage, and found that there was a maximum number of births on the 275th day, then a decrease and a second maximum on the 293rd day. Nevertheless, in spite of careful reckoning, there are, as will be recognized, many sources of error, and medical men and nurses are often wisely cautious of giving any exact date for an anticipated birth; sometimes too cautious even to suggest the week within which the birth will take place. I have known a good many mothers, however, who were much more accurately certain about this point than their attendants, and have found that the birth took place exactly on the day they anticipated. As an illustration of this, I give the answer from one of my correspondents, both of whose children were born on the exact day she anticipated. I asked her how she estimated these periods, and she said:--