Since the inclination to empty the bladder more often than usual may be due merely to nervousness or to many other conditions, this symptom taken alone cannot be regarded as a definite sign of pregnancy. Indeed, it is mentioned, not because of its importance, but to point out that it is in no way connected with the kidneys, as patients are sometimes led to believe. It is a direct and natural result of pregnancy. Since the womb enlarges and tilts forward at a more acute angle than formerly, it presses against the bladder, giving the same sensation as when the bladder is distended with urine. 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.