Placenta previa is an obstetric complication that occurs in the second
and third trimesters of pregnancy. It may cause serious morbidity and
mortality to both the fetus and the mother. It is one of the leading
causes of vaginal bleeding in the second and third trimesters.
Placenta previa is generally defined as the implantation of the placenta over or near the internal os of the cervix.
• Total placenta previa occurs when the internal cervical os is completely covered by the placenta.
• Partial placenta previa occurs when the internal os is partially covered by the placenta.
• Marginal placenta previa occurs when the placenta is at the margin of the internal os.
• Low-lying placenta previa occurs when the placenta is implanted in the
lower uterine segment. In this variation, the edge of the placenta is
near the internal os but does not reach it.
• A recent study concluded that more than two thirds of women with a
distance of more than 10 mm from the placental edge to cervical os have
vaginal delivery without an increased risk of hemorrhage
Pathophysiology
The exact etiology of placenta previa is unknown. The condition may be
multifactorial and is postulated to be related to multiparity, multiple
gestations, advanced maternal age, previous cesarean delivery,2 previous
abortion, and possibly, smoking. Unlike first trimester bleeding,
second and third trimester bleeding is usually secondary to abnormal
placental implantation.
Causes
• Prior uterine insult or injury
• Risk factors
o (Prior placenta previa (4-8%
o First subsequent pregnancy following a cesarean delivery
o (Multiparity (5% in grand multiparous patients
o Advanced maternal age
o Multiple gestations
o Prior induced abortion
o Smoking