Although the presumptive signs which we have considered by no means
exhaust the list, all the others are totally untrustworthy. Each of
the more reliable symptoms, as we have seen, must be accepted
cautiously; but taken altogether, except in very unusual cases, they
may be relied upon. _If, for example, menstruation has previously
been regular and then a period is missed, the patient has good reason
to suspect she is pregnant; if the next period is also missed and
meanwhile the breasts have enlarged, the nipples darkened, and the
secretion of colostrum has begun, it is nearly certain that she is
pregnant; whether morning sickness and the desire to pass the urine
frequently are present is of no importance._ But the most
characteristic evidence, we must remember, is not available until the
eighteenth or twentieth week; then the signs of pregnancy are
unmistakable.

THE DURATION OF PREGNANCY.--After the existence of pregnancy has
become assured, perhaps the greatest interest centers about the date
upon which the birth may be expected. Even to approach accuracy in
this prediction the prospective mother must be familiar with certain
facts which she will always observe, but which, unless she
appreciates their importance early in pregnancy, she may fail to
record or to remember. In a few cases, however, such exceptional
information as knowing the date of conception does not lead to an
absolutely accurate prediction. But the deviation from the rule will
be understood only after we understand the rule itself, which is
based upon what we accept as the average duration of human pregnancy,
technically called the period of gestation.

In a broad sense, the period of gestation for each variety of mammal