Resolution of a Low-Lying Placenta and Placenta Previa Diagnosed at the Midtrimester Anatomy Scan

Placenta previa or low-lying placenta is identified on a significant number of midtrimester ultrasounds (USs), and the large majority of these resolve by term. While guidelines recommend follow-up US at 32 weeks' gestation to assess for resolution of midtrimester previa, limited data exist to support this recommendation. This study sought to determine the incidence and rate of resolution of a low-lying placenta or placenta previa identified in the midtrimester. The authors sought to assess the optimal time to perform follow-up US in the third trimester to assess for resolution and to identify factors associated with resolution of placenta previa and a low-lying placenta. The authors conducted a retrospective cohort study of all patients with a diagnosis of a low-lying placenta or placenta previa at the time of anatomy screening at a single institution between February 12, 2010, and April 30, 2015. Patients were eligible if they had a low-lying placenta or placenta previa with continuation of pregnancy beyond viability (defined as a gestational age of >24 weeks 0 days). Fetal anatomy surveys were performed between 18 weeks 0 days and 23 weeks 6 days. The placental location in relation to the maternal cervix was assessed by US for all patients. Transvaginal US was performed by trained sonographers, measuring the distance from the leading placental edge to the internal cervical os. Patients with a leading placental edge of greater than 0 mm but 20 mm or less from the internal cervical os were diagnosed with a low-lying placenta, whereas patients with a leading placental edge overlapping the internal cervical os were diagnosed with placenta previa. The study found that a low-lying placenta or placenta previa was diagnosed in 8.7% of women at the midtrimester anatomy survey when using transvaginal US. Although the incidence in the population was high, most women who returned for additional assessments had resolution, and 95% of resolutions occurred by 17 weeks from diagnosis (corresponding to 36 weeks' gestation in the cohort). The distance from the internal cervical os at the time of the midtrimester anatomy survey and multiparity were predictors of resolution, whereas prior cesarean delivery was not. In all, 91.9% (95% confidence interval, 90.2%-93.3%) of women with a diagnosis of a low-lying placenta or placenta previa at the midtrimester anatomy survey had resolution before delivery. The median time to resolution was 10 weeks from diagnosis (corresponding to 29 weeks' gestation in the cohort), with a 95th percentile of 17 weeks (corresponding to 36 weeks' gestation in our cohort). The chance of resolution was inversely proportional to the distance of the placental edge from the internal os and was 99.5% for women with the placental 10 and 20 mm from the internal os and 72.3% for women with a placenta previa. Resolution of low lying placenta occurs in most cases by the third trimester. Optimal timing of follow-up US should balance the potential benefit of diagnosing resolution at an earlier gestational age with the possible need for multiple additional US assessments.