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Emergency And Acute Medicine – Placenta Previa
Placenta previa is defined as placental tissue overlying or positioned close to the internal cervical os. As the uterus enlarges and the cervix begins to dilate, placental vessels near the cervix may tear, resulting in vaginal bleeding. More than 90% of placenta previa diagnosed before 20 weeks’ gestation will resolve as the placenta “migrates” upward with uterine growth. However, if the placenta overlaps the internal os by more than 20 mm, previa is highly likely to persist at delivery. Greater degrees of overlap (15–23 mm or more) strongly predict persistence at term. Placenta previa accounts for approximately 20% of antepartum hemorrhage cases.
Placenta previa is classified into four types: complete (placenta completely covers the cervical os), partial (partially covers the os), marginal (placental edge reaches the margin of the os), and low-lying placenta (placental edge lies within 2 cm of the os). The overall incidence at term is approximately 0.4% of pregnancies. Maternal mortality is low (around 0.03%), but perinatal morbidity and mortality are increased—largely due to preterm delivery.
The exact etiology is unknown. Factors affecting implantation location include prior uterine curettage, abnormal endometrial vascularization, and delayed ovulation. Risk factors include multiparity (especially grand multiparity), multiple gestation, prior cesarean section (risk increases with number of previous C-sections), advanced maternal age, prior placenta previa, smoking, assisted reproduction, Asian maternal race, high-altitude residence, male fetus, and elevated maternal serum alpha-fetoprotein. Placenta previa is also associated with congenital anomalies, abnormal fetal presentation, preterm premature rupture of membranes, and amniotic fluid embolism. Placenta accreta spectrum disorders (accreta, increta, percreta) occur in 5–10% of patients with previa and may require cesarean hysterectomy due to severe bleeding.
The hallmark presentation is painless bright red vaginal bleeding after 20 weeks’ gestation. Seventy percent of patients present with painless bleeding, while about 20% may have associated uterine contractions. The first bleeding episode typically occurs between 27 and 32 weeks. Bleeding may range from minor spotting to massive hemorrhage, and recurrence is common. The severity or number of bleeding episodes does not necessarily correlate with the degree of placental coverage. Intercourse or heavy exercise may precipitate bleeding, though often there is no clear inciting event.
On examination, digital vaginal examination must never be performed in second- or third-trimester bleeding until placenta previa has been excluded by ultrasound, as this may precipitate severe hemorrhage. A sterile speculum exam is safe and may help identify whether bleeding originates from the cervical os, vagina, or another lesion. Signs of significant bleeding include blood pooling at the patient’s feet and vital sign instability such as tachycardia or hypotension. Continuous fetal heart rate monitoring is essential.
Diagnosis is made primarily by ultrasound. Transabdominal ultrasound is 93–98% accurate but may have false negatives (e.g., obesity, posterior placenta) and false positives (e.g., overdistended bladder). If placenta previa is suspected or findings are uncertain, transvaginal ultrasound should be performed, as it is essentially 100% accurate and does not increase bleeding risk. Color Doppler ultrasound may help identify placenta accreta. MRI can assist in evaluating invasive placental disorders.
Laboratory evaluation includes CBC, platelets, type and screen (or cross-match if transfusion anticipated), and Rh status. Kleihauer–Betke testing is performed in Rh-negative patients to detect fetomaternal hemorrhage. Coagulation studies are obtained if coagulopathy is suspected.
Prehospital management involves transport to a facility capable of managing high-risk or preterm deliveries. If hypotensive, the patient should be positioned in the left lateral recumbent position. Oxygen and IV access should be established. Initial stabilization includes airway, breathing, and circulation assessment, two large-bore IV lines, crystalloid resuscitation, and blood transfusion as needed. Blood transfusion is indicated for significant hypotension or hematocrit less than 30%. Fresh frozen plasma may be required for coagulopathy. Continuous fetal monitoring and immediate obstetric consultation are mandatory for symptomatic patients.
In the emergency department, patients with active bleeding require emergent obstetric consultation. Maintain NPO status and bed rest until obstetrics determines stability. Rh-negative patients should receive Rho(D) immune globulin (300 μg IM), with additional dosing guided by Kleihauer–Betke results. Magnesium sulfate may be used for preterm contractions when delivery is not indicated. Antenatal corticosteroids (e.g., betamethasone 12 mg IM every 24 hours for two doses) are recommended between 24 and 34 weeks to promote fetal lung maturity. Emergency cesarean delivery is indicated for ongoing hemorrhage or fetal compromise.
All patients with active bleeding from placenta previa should be admitted, as this condition constitutes a potential obstetric emergency. Selected stable patients whose bleeding has resolved may be managed outpatient in consultation with obstetrics. Asymptomatic patients with incidental findings may not require admission but should follow strict instructions, including pelvic rest and prompt reporting of any bleeding or contractions. If the placenta overlies the os by more than 20 mm, cesarean delivery is typically planned at 36–37 weeks.
Painless vaginal bleeding after 20 weeks’ gestation should be considered placenta previa until proven otherwise, whereas painful vaginal bleeding suggests placental abruption. Importantly, both conditions can coexist. Digital vaginal examination must be avoided until previa is excluded, while sterile speculum examination and transvaginal ultrasound are safe and appropriate diagnostic tools.
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