Pregnancy Defined.—­ Pregnancy begins with conception and ends with parturition; it provides for the nutrition and the expulsion of the embryo and for its nutrition for a short time after birth.

The average duration of pregnancy is ten lunar months, or two hundred and eighty days.  The date of the confinement is calculated by reckoning from the date of the last menstrual flow; count backward three months from the date of the first appearance of the last menses; to this add twelve months and seven days, five days being for the average menstrual duration and two days for the possibility of fecundation.

Duration of Pregnancy.—­ Many difficulties are experienced in determining the date of the expected confinement.  As most pregnancies occur in married women, we cannot base any calculations on a single act of coitus.  And even if there was but one, all physiologists agree that there is a variable period in different women, and in the same woman at different times, between insemination and the fertilization of the ovum.  It is the moment of fecundation, or the union of the germ and sperm cells, which marks the beginning of pregnancy.  The uncertainty becomes still greater owing to our inadequate knowledge as to the length of time during which the sexual elements, the ova and the spermatozoa, retain their vitality after liberation from their respective sources.  While it is not certainly known, it is probable that the ovum is capable of impregnation any time during its sojourn within the oviduct and before reaching the uterus, or probably for a period of about one week from the time of its escape from the Graafian follicle.  The remarkable vitality of the spermatozoa even under less favorable circumstances—­ direct observation shows that these elements retain their movements for over nine days outside of the body—­ renders it almost certain that their powers of fertilization are maintained for a long time after they are deposited within the healthy female genital tract; it is believed that the spermatozoa are capable of fertilization after a sojourn of three or more weeks within the oviduct.

Consideration of these facts renders apparent the impossibility of fixing with certainty the date of the beginning of pregnancy, since conception may result from the union of the ovum liberated at the beginning of the period with the spermatozoon introduced at the end of that time; or it may result from the meeting of the male elements already within the oviduct with an ovum discharged a day or two before the occurrence of the menstrual period.

The Signs of Pregnancy.—­ The cessation of the menstrual period is the sign of the greatest value in women who have been regular; but it must always be remembered that there may be an irregularity of menstruation for the first few months after marriage.  The appetite is capricious; morning sickness or nausea in the morning on first getting up is a very common symptom in the early months of pregnancy; enlargement