Understanding Placenta Previa: Types, Risks, and Management in Pregnancy

Placenta previa is an obstetric condition in which the placenta implants in the lower segment of the uterus, partially or completely covering the internal cervical os. This abnormal placement can lead to bleeding and complications as the cervix begins to dilate in the second or third trimester.

Classification
Placenta previa is classified into four main types based on how much of the cervical opening is covered:

• Complete placenta previa: The placenta entirely covers the cervical os, making vaginal delivery impossible.
• Partial placenta previa: The placenta covers only part of the cervical os.
• Marginal placenta previa: The edge of the placenta reaches the margin of the cervix without covering it.
• Low‑lying placenta: The placenta implants in the lower uterine segment but remains at least 2 cm away from the cervical os.

Global Incidence
Placenta previa occurs in approximately 0.3–0.5% of all pregnancies, though rates may be higher in populations with elevated cesarean delivery rates or advanced maternal age.

Trends
Over recent decades, the incidence of placenta previa has increased slightly, correlating with the rise in repeat cesarean deliveries and assisted reproductive technologies.

Previous Cesarean Delivery
Women with one or more prior cesarean sections have up to a four‑fold increased risk of placenta previa, especially when the placenta implants over the scar site.

Multiple Pregnancies
Carrying twins, triplets, or higher‑order multiples stretches the uterine lining and predisposes to low implantation of the placenta.

Advanced Maternal Age
Pregnancies in women aged 35 and older show a higher incidence of placenta previa, possibly due to age‑related changes in uterine vasculature.

Multiparity
Having had several prior births increases the risk, likely because of repeated endometrial remodeling.

Smoking
Tobacco use is linked to placental abnormalities, including previa, via its effects on uterine blood flow.

Uterine Surgery or Instrumentation
Procedures such as dilation and curettage or myomectomy can alter the endometrial lining, favoring low placental implantation.

Painless Vaginal Bleeding
The hallmark symptom is sudden, painless bleeding in the second or third trimester. Bleeding may be scant or profuse and often recurs.

Timing of Bleeding
Episodes typically begin after 20 weeks’ gestation, with the risk of hemorrhage increasing as the lower uterine segment thins.

Hemodynamic Status
While many women remain hemodynamically stable, severe cases can lead to hypovolemic shock, necessitating urgent intervention.

Ultrasound Evaluation
Ultrasound is the diagnostic gold standard. Transabdominal scans can suggest low placental location, but transvaginal ultrasound provides more precise measurement of the placental edge relative to the cervical os.

Transabdominal Ultrasound
Often used initially; may be limited by maternal habitus or fetal position.

Transvaginal Ultrasound
Safe after 20 weeks’ gestation and more accurate for determining the exact distance between placenta and os.

Magnetic Resonance Imaging (MRI)
Reserved for complex cases, especially when placenta accreta spectrum is suspected or when ultrasound findings are inconclusive.

Expectant Management
In stable patients with minimal bleeding, outpatient observation with pelvic rest (no intercourse or digital exams) and scheduled ultrasounds may suffice.

Hospitalization
Recurrent or heavy bleeding, preterm labor, or distance from a tertiary care center warrant inpatient monitoring for timely intervention.

Blood Transfusion
Cross‑matched blood should be available for patients at risk of significant hemorrhage; transfusion thresholds follow institutional protocols.

Medications
Corticosteroids may be administered to enhance fetal lung maturity if preterm delivery (<34 weeks) is anticipated. Tocolytics are generally avoided due to risk of masking bleeding.

Timing and Mode of Delivery

• Cesarean Section Planning: Complete and partial previa require scheduled cesarean delivery, often at 36–37 weeks if bleeding is controlled.
• Vaginal Delivery: Marginal or low‑lying placenta previa with minimal bleeding may allow trial of labor under strict monitoring.

Maternal Risks

• Hemorrhage: Leading cause of maternal morbidity, potentially requiring hysterectomy.
• Placenta Accreta Spectrum: Abnormal adherence of the placenta to the myometrium occurs more frequently with previa, especially over a scar.
• Infection and Thrombosis: Prolonged hospitalization increases risks of infection and venous thromboembolism.

Fetal Risks

• Preterm Birth: Early delivery is common, with associated neonatal respiratory distress and intensive care needs.
• Growth Restriction: Chronic placental insufficiency can impair fetal growth.
• Hypoxia: Acute hemorrhage may compromise uteroplacental perfusion.

Prenatal Care
Early and regular antenatal visits allow timely identification of risk factors and ultrasound screening for placenta location.

Risk Factor Modification
Encourage smoking cessation and counsel on spacing pregnancies to reduce multiparity risk.

Education and Birth Planning
Discuss signs of bleeding, the importance of immediate medical attention, and delivery planning at a facility equipped for high‑risk obstetrics.