In order to understand what is happening to the patient in this condition, we must go back to the question of the implantation of the ovum. We learned that, as a rule, after the ovum entered the uterus it attached itself to a point in the uterine lining high up on the anterior or posterior wall. Unhappily, the position of this point of attachment is a mere matter of chance, and the ovum sometimes, but not often, is implanted so far down toward the cervix that as the placenta develops at that site it partially or completely overlaps the internal os. It is the extent to which the placenta grows over the cervical opening that determines whether it is of the central, partial or marginal variety. [Illustration: FIG. 45.—Partial placenta prævia. Section of uterine wall and cervix showing that part of the maternal surface of the placenta which extends over the cervical opening and is exposed by dilation of the internal os, with an escape of blood from the open vessels as a result. Drawn by Max Brodel. (From “The Treatment of Placenta Praevia,” by William B. Thompson, M.D.—Johns Hopkins Hospital Bulletin, July, 1921.) ] _A centrally implanted placenta prævia_ (Fig. 44) is one which entirely covers the os; _a partial placenta prævia_ (Fig. 45), as the name suggests, only partially covers the opening, while if it is implanted so high up that only its margin overlaps the os, it is designated as _marginal placenta prævia_. (Fig. 46.) [Illustration: FIG. 46.—Diagram of marginal placenta prævia. ] Another classification groups all placenta prævia as complete or incomplete, the latter comprising the partial and marginal varieties, as well as the lateral which is so attached that it does not quite reach the edge of the internal os. However, as these terms do not differ widely and are clearly descriptive, the differences are of no great moment to the nurse, as the treatment is practically the same and the nurse’s duties quite the same for all varieties. =Cause.= Not much is definitely known about the cause of placenta prævia, but it is evident that multiparity is a factor, since the condition is found about six times as frequently among women who have borne children, as it is among those who are pregnant for the first time. A diseased uterine lining is probably the fundamental cause, and this may explain why the trouble is found more frequently among the poorer classes, since such women as a class have less skilled medical attention than those in better circumstance. One theory is that an old endometritis results in a very unfertile soil for the implantation of the ovum and as a result the ovum migrates to other parts of the uterine cavity in its search for a more favorable site, and comes to lodge near the lower segment.