Pathophysiology:
The women at highest risk are those with prior placenta previa or multiple prior cesarean sections. The strong association between placenta previa and parity has suggested that "endometrial damage" is an etiologic factor. Presumably, each pregnancy "damages" the endometrium underlying the implantation site, rendering the area unsuitable for implantation. Subsequent pregnancies are more likely to become implanted in the lower uterine segment by a process of elimination. This effect is most clearly seen with prior term pregnancies, but multiple early pregnancy terminations may also be related to an increased incidence of placenta previa. There is evidence that low implantations are much more common early in pregnancy, but that the great majority of these "resolves" and never become symptomatic. With the progression of pregnancy, more than 90% of these low-lying placentas identified early in pregnancy will appear to move away from the cervix and out of the lower uterine segment. Although the term "placental migration" has been used, most authorities do not believe the placenta moves. Rather, it is felt the placenta grows preferentially toward a better vascularized fundus (trophotropism), whereas the placenta overlying the less well vascularized cervix may undergo atrophy (2).