What distance from os requires cesarean delivery?

Placental edge <20 mm from internal os at ≥28 weeks has high likelihood of requiring cesarean (vaginal delivery attempted for 20-40 mm in select cases with favorable findings). Complete previa (os covered) always requires cesarean.

When is transabdominal vs transvaginal ultrasound preferred?

Transvaginal ultrasound is safe and more accurate for placental localization near the os than transabdominal. AIUM/ACOG recommend TVS for all suspected low-lying placentas regardless of transabdominal findings.

Do low-lying placentas resolve with advancing gestation?

Yes. Most low-lying placentas at 16-20 weeks resolve by 28 weeks as the lower uterine segment lengthens. Only 10% of low-lying placentas at 20 weeks persist as previa at term. Repeat scan at 28-32 weeks is recommended.

Placenta Previa Distance from Internal Os Classification

Classify placenta previa by placental edge distance from the internal cervical os on transvaginal ultrasound. Distance <20 mm with posterior low-lying placenta or complete previa requires cesarean. Guides delivery planning.

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