Placenta Previa

Introduction

• Description: Placenta previa is the implantation of the placenta in a location that leaves a part or all of the cervical os covered. This is associated with potentially catastrophic maternal bleeding and obstruction of the uterine outlet. Several degrees are recognized: total, partial, marginal, and low-lying placenta. These degrees may vary with cervical dilation or gestational age.

• Prevalence: Observed in 0.3%–0.5% of deliveries; up to 6% of patients at 10–20 weeks gestation.

• Predominant Age: Reproductive age; average age is 29 years.

• Genetics: No genetic pattern.

Etiology and Pathogenesis

• Causes: Implantation by the zygote low in the uterine cavity (in close proximity to the cervical os). Defective decidual vascularization, resulting from inflammation or atrophy, has been implicated.

• Risk Factors: Multiparity, advanced maternal age (>35 years: 1% of deliveries, >40 years: 2%), prior cesarean delivery (two to five times increase based on the number of procedures), induced abortion, smoking (2-fold increase), cocaine use, multiple gestation (40% increased risk), male fetus, high altitude, prior placenta previa, and prior abortion.

Signs and Symptoms

• Asymptomatic; found on routine ultrasonography (90% of cases found on routine ultrasonography before 20 weeks of gestation will resolve before delivery), 10% reach term without symptoms

• Painless vaginal bleeding (70%–90%; generally not present until late second or early third trimester), which may be catastrophic in amount, although initial episodes are rarely fatal; blood is maternal in origin. Vaginal bleeding at more than 20 weeks gestation precludes vaginal examination and should prompt immediate ultrasonography to evaluate placental location and health.

• Uterine hyperactivity possibly present with bleeding (20%)

• Fetal malpresentation

• Heavy or prolonged bleeding after delivery (20%–25%)

Diagnostic Approach