From the Guidelines
For patients with placenta previa, ultrasounds are typically performed every 2-4 weeks, with more frequent monitoring in the third trimester, particularly at 28-32 weeks as recommended by the Society for Maternal-Fetal Medicine 1. The frequency of ultrasounds for placenta previa can vary depending on the patient's symptoms and the presence of other complications such as placenta accreta spectrum disorder (PASD).
- Asymptomatic patients with placenta previa may undergo weekly or biweekly ultrasound measurements to predict antepartum bleeding and need for early cesarean delivery 1.
- The imaging will be driven by patient symptoms, such as vaginal bleeding, as well as by delivery planning, with follow-up US imaging useful to assess for interval change and possible progression of the depth of invasion 1. Key factors that influence the frequency of ultrasounds include:
- Patient symptoms, such as vaginal bleeding
- Presence of uterine scar or "low-lying" placenta over the uterine scar
- Need for delivery planning and timing
- Suspicion of placenta accreta or other placental invasion disorders The purpose of these ultrasounds is to monitor the position of the placenta relative to the cervix, assess for placental invasion disorders, evaluate fetal growth, and plan for appropriate timing and mode of delivery, typically via cesarean section 1.
From the Research
Ultrasound Frequency for Placenta Previa
- The frequency of ultrasounds for placenta previa varies, but studies suggest that serial ultrasounds are performed every 2 weeks until delivery or placental migration for more than 3 cm from the internal cervical os 2, 3.
- In one study, ultrasounds were performed at 28-32 weeks of gestation, and then repeated every 2 weeks until delivery or placental migration 2.
- Another study found that most patients had a 28-week scan, and even when clearance occurred, most patients had a repeat ultrasound at 32 weeks 4.
- The shape of the lower placental edge may also predict clinical outcome, with thick-edge low-lying placenta having a higher rate of antepartum hemorrhage and abdominal delivery 5.
Factors Influencing Ultrasound Frequency
- The degree of placenta previa, placental position, and relation to the presenting part may influence the frequency of ultrasounds 2, 3.
- Pelvic rest recommendations do not appear to affect the timing of repeat imaging or delivery 4.
- The use of transvaginal sonography may improve the prediction of mode of delivery and clinical outcome 3, 5.
Clinical Implications
- Ultrasounds are important for the diagnosis of placental localization and placental migration during the third trimester 2.
- Placental migration takes place more often in anterior than in complete or posterior placenta previa 2, 3.
- The management of placenta previa may involve a multidisciplinary approach, including ultrasound evaluation, hospitalization, antenatal corticosteroids, and scheduled cesarean section 6.