Follow-Up Ultrasound Timing for Low-Lying Placenta at 28 Weeks
A low-lying placenta 1.7 cm (17 mm) from the internal os at 28 weeks requires repeat ultrasound evaluation at 32 weeks gestation to reassess placental position and guide delivery planning. 1
Recommended Follow-Up Protocol
Timing of Next Ultrasound
- Schedule follow-up ultrasound at 32 weeks gestation to re-evaluate the evolving relationship between the placental edge and internal cervical os 2, 1
- The American College of Radiology recommends third trimester follow-up (28-32 weeks) for all low-lying placentas to reassess placental position 1
- Recent evidence suggests that most resolutions occur by 28 weeks, with median time to resolution of 10 weeks from initial diagnosis, making 32-week imaging optimal for final assessment 3, 4
What to Assess at Follow-Up
The follow-up ultrasound should evaluate:
- Placental edge-to-internal os distance using transvaginal ultrasound for precise measurement 2, 1
- Placental location and morphology, including assessment for any concerning features of placenta accreta spectrum disorder 2
- Cervical length to assess preterm delivery risk 2
- Doppler evaluation to identify any vessels overlying the internal cervical os (vasa previa) 2, 1
Risk Stratification at 1.7 cm Distance
Resolution Likelihood
- At 17 mm from the internal os, your patient has a high probability of resolution (approximately 95-99%) based on research showing that placentas 10-20 mm from the os resolve in 99.5% of cases 3
- However, posterior placentas have 2.4 times higher risk of persistence compared to anterior placentas, so placental location matters 5
- The median time to resolution from mid-trimester diagnosis is 9-10 weeks, meaning most cases clear by 28-30 weeks 3
Special Considerations Based on Risk Factors
Prior cesarean delivery significantly increases risk:
- Women with previous cesarean sections have 3.7 times higher risk of persistent low-lying placenta at term 5
- If anterior placenta with prior cesarean, carefully evaluate for placenta accreta spectrum disorder features including loss of retroplacental clear space, placental lacunae, and bladder wall abnormalities 2, 1
Management Between Now and Follow-Up
Activity Recommendations
- After 28 weeks, limit to low-intensity activities such as walking and avoid moderate-to-vigorous physical activity 6
- Avoid travel requiring flights longer than 2-3 hours or destinations more than 30-60 minutes from appropriate medical facilities 6
Precautions
- Avoid digital pelvic examination until placenta previa is definitively excluded to prevent triggering hemorrhage 1
- Maintain awareness that any bleeding episode increases risk for subsequent hemorrhage 6
- Ensure patient carries complete prenatal records including ultrasound measurements 6
Delivery Planning Considerations
If Placenta Clears at 32 Weeks (Most Likely Scenario)
- If placental edge is ≥2 cm from internal os at 32 weeks, the patient can proceed with routine obstetric care and vaginal delivery planning 1
If Placenta Remains Low-Lying
- Notify blood bank due to increased hemorrhage risk and ensure adequate blood products available 1
- Consider multidisciplinary team approach if any features concerning for placenta accreta spectrum 1
- Trial of labor may be considered for low-lying placenta (not previa) in select cases, particularly if no marginal sinus present, though intrapartum blood loss averages 946 mL 7
Common Pitfalls to Avoid
- Failing to use transvaginal ultrasound for accurate measurement—transabdominal imaging alone is insufficient for precise placental edge-to-os distance 1
- Missing placenta accreta spectrum in women with anterior low-lying placenta and prior cesarean deliveries—risk increases 7-fold after one cesarean to 56-fold after three cesareans 1, 6
- Scheduling follow-up too late—while 32 weeks is standard, some cases may benefit from 28-30 week imaging if clinical concerns arise 4
- Performing digital examination before excluding placenta previa, which can trigger life-threatening hemorrhage 1