On obstetric ultrasound at 20 weeks, the placenta appears to completely cover the internal os. On repeat ultrasound at 32 weeks, the lower edge of the placenta is 2 cm from the internal os. The correct interpretation is:

• A Placenta previa — delivery by cesarean section is mandatory
• B Marginal placenta previa — management is expectant
• C Low-lying placenta — vaginal delivery may be considered with caution ✓
• D Normal placentation — no special management required

Explanation

A lower placental edge 0.1–2.0 cm from the internal os at ≥32 weeks is classified as 'low-lying placenta.' Placenta previa is defined as placental edge at or covering the internal os. Low-lying placenta may permit vaginal delivery if the edge is >2 cm from the os at term, but 2 cm requires careful obstetric assessment. Apparent placenta previa seen in the second trimester often resolves with the 'migration' phenomenon (differential uterine growth), which is why repeat ultrasound at 32–36 weeks is standard. Complete previa at this gestation would mandate cesarean section.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.