day of conception. As this date is seldom known, it is most
convenient to reckon from the first day of the last menstrual period.
Estimated in this way its average duration is 280 days. As this
period corresponds to ten menstrual cycles, physicians prefer to
describe pregnancy as lasting 10 lunar months of four weeks each.
This is equivalent to 9 calendar months, in terms of which its
duration is popularly stated.

THE ESTIMATION OF THE DATE OF CONFINEMENT.--Since pregnancy is not an
absolutely fixed period, we possess no reliable means of predicting
the exact day when it will end. The most satisfactory method of
prediction consists in counting forward 280 days from the beginning
of the last menstruation or, what gives the same result, counting
backward eighty-five days from this date. _To make the calculation
in the simplest way we count back three months and add seven
days_; this addition is made because seven days generally
represents the difference between three months and eighty-five days.
If the last menstruation, for example, began on October 30th, we
count back three months to July 30th and add seven days, which gives
August 6th as the probable date of confinement.

A prospective mother should remember that this prediction is no more
than approximate. The calculation does not give the exact date of
delivery more than four or five times in a hundred cases. It is
accurate within a week in half the cases and within two weeks in
four-fifths. We also know that delivery is somewhat more likely to
occur after the expected date than before it. But perhaps we shall
get the clearest idea of the accuracy of the rule, or better still
of its inaccuracy, if we imagine twenty patients to have the same
predicted date, all of them giving birth to mature infants. The