Proposal of a new parameter for diagnosing a low-lying placenta in the third trimester: The distance between the external os and placental edge

Abstract Objective To validate a new parameter of the distance between the external os (EO) and placental edge (PE) to diagnose a low-lying placenta in the third trimester. Materials and methods The study participants included 94 uncomplicated singleton pregnant women with cephalic presentation. These women were cared for in our hospital in 1998–2011, with a posterior low-lying placenta, which was diagnosed as the distance between the internal os (IO) and a PE of 0–3.0 cm at 34–36 weeks' gestation. Measurements of cervical length (CL) and the distances of IO–PE and EO–PE were performed using transvaginal ultrasonography at least twice at 28–30 weeks, 31–33 weeks, and 34–36 weeks. Changes in CL, and the IO–PE and EO–PE distances were analyzed. Results CL and the IO–PE and EO–PE distances did not change prior to 31–33 weeks. CL was shortened and the IO–PE distance was increased after 31–33 weeks ( p  = 0.0001), but the EO–PE distance was unchanged. Conclusion The EO–PE distance is a promising parameter for diagnosis of low-lying placenta in the third trimester up to 36 weeks' gestation.