These conditions are relatively infrequent, and each happens about once in 200 pregnancies. They usually occur in the last trimester, and the predominant symptom in both is vaginal hemorrhage. In placenta praevia, the placenta, instead of being at the top of the uterus, is situated low down, and part of it overlaps the mouth of the womb (internal os). The uterus is shaped like a large inverted bottle, and ordinarily the placenta is implanted high upon the sides. In the case of placenta praevia, however, it is situated so low down that part, or all, of the round opening from which the bottleneck arises is covered by it. Late in pregnancy, preparatory to the onset of labor, the cervix, the neck of the bottle in our analogy, expands slightly and any placental tissue that overlies it is torn loose, leaving an area from which the patient bleeds. Placenta praevia is rare in a first pregnancy and increasingly more common with additional gestations.
Premature separation of the placenta (frequently termed placental abruption), as the name suggests, is the separation of part or all of the placenta from its attachment to the uterus before the child is born. A localized hemorrhage begins in the maternal tissues underlying the placenta; blood accumulates and sheers loose the placenta from its delicate, spongelike fastenings. Premature separation occurs most frequently in those who have borne more than seven children; about 40 per cent of the cases show high blood pressure and albumin in the urine, suggesting the association with a generalized toxic process. Also very occasionally a placenta is jarred loose by a severe abdominal blow.