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

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.