What is the difference between placenta previa and low lying placenta in a pregnant woman?

Placenta Previa vs Low-Lying Placenta: Key Differences

Placenta previa occurs when the placenta completely overlies the internal cervical os, while a low-lying placenta is diagnosed when the placental edge is within 2 cm of the internal os but does not cover it. 1

Diagnostic Definitions

Placenta Previa

• The placenta covers or overlaps the internal cervical os 1, 2
• Requires cesarean delivery at 34 0/7 to 35 6/7 weeks for uncomplicated cases 3
• Carries significantly higher risk of persistence into third trimester (20.4% vs 1.4% for low-lying placenta) 4

Low-Lying Placenta

• Placental edge is within 2 cm of the internal os but does not cover it 1, 2
• If the placental edge is >2 cm but within 3.5 cm from the internal os, some sources term this "low-lying" 2
• Vaginal delivery may be attempted if the placental edge is >5 mm from the internal os in the late third trimester 5
• Resolution rate is much higher than placenta previa (98.6% vs 79.6%) 4

Clinical Significance of the Distinction

Risk Stratification

• Women with placenta previa in the second trimester have an 18-fold higher risk of persistent abnormal placentation at delivery compared to those with low-lying placenta 4
• Posterior placentas carry 2.4 times higher risk of persistence than anterior placentas 4
• Prior cesarean delivery increases risk 3.7-fold for persistent abnormal placentation 4

Delivery Planning Differences

For Placenta Previa:

• Mandatory cesarean delivery 3, 2
• Delivery at level III or IV maternal care facility with multidisciplinary team 3
• Blood bank notification required due to hemorrhage risk 3
• Delivery timing at 34-36 weeks to balance neonatal outcomes against maternal bleeding risk 3

For Low-Lying Placenta:

• Trial of labor is appropriate if internal-os-distance >5 mm 5
• Cesarean section performed in approximately one-third of cases due to bleeding if placenta within 2 cm of os 6
• Increased vigilance for postpartum hemorrhage even with vaginal delivery 2

Diagnostic Approach

Gold Standard Imaging

• Transvaginal ultrasound is the gold standard for both conditions, with 90.7% sensitivity and 96.9% specificity 3
• Transabdominal ultrasound should be performed first, followed by transvaginal for precise measurement 1
• Digital pelvic examination must be avoided until placenta previa is excluded, as it can trigger life-threatening hemorrhage 1, 3

Measurement Technique

• Measure the internal-os-distance (IOD) - the distance from the placental edge to the internal cervical os 1, 4
• Use a moderately full bladder (200-300 mL) for optimal visualization 1
• High-frequency linear probe (5-9 MHz) provides best resolution 1

Follow-Up Protocol

Timing of Reassessment

• Initial diagnosis typically at 18-22 week anatomy scan 1, 3
• Follow-up ultrasound at 28-32 weeks is essential to reassess placental position 1, 3
• Additional scan at 32-34 weeks if previa persists 3

When Follow-Up Can Be Avoided

• If the placenta is ≥2 cm from the internal os at 18-20 weeks, follow-up evaluation is not necessary 1
• For anterior low-lying placenta, the IOD cut-off for requiring follow-up can be lowered from 20 mm to 5 mm without missing high-risk cases 4

Resolution Patterns

• Overall resolution rate for low-lying placenta/previa diagnosed at mid-trimester is 91.9% 7
• Median time to resolution is 10 weeks from diagnosis 7
• Resolution probability by distance: 99.5% for 10-20 mm, 95.4% for 0.1-10 mm, and 72.3% for placenta previa 7

Special Considerations for Placenta Accreta Spectrum

High-Risk Scenarios

• All women with placenta previa or low-lying anterior placenta AND prior cesarean deliveries must be evaluated for placenta accreta spectrum disorder 1, 3
• Risk increases 7-fold after one cesarean to 56-fold after three cesareans 1, 3
• Up to 40% of women with placenta previa and three prior cesareans will develop placenta accreta spectrum 8

Ultrasound Features Suggesting Accreta

• Placental lacunae (sonolucent spaces) - highest sensitivity finding 1
• Loss of retroplacental clear space 1
• Myometrial thickness <1 cm 1
• Bladder wall abnormalities 1
• Abnormal Doppler flow patterns with increased subplacental vascularity 1

Common Pitfalls to Avoid

• Failure to perform transvaginal ultrasound for accurate IOD measurement 1
• Performing digital examination before excluding placenta previa 1, 3
• Missing placenta accreta spectrum in women with anterior low-lying placenta and prior cesarean deliveries 1
• Applying excessive probe pressure during transabdominal evaluation, which creates artifactual loss of the retroplacental zone 1
• Evaluating with an empty bladder, which impedes assessment of the lower uterine segment 1

Activity and Travel Restrictions

• Women with asymptomatic low-lying placenta or previa can continue moderate-to-vigorous physical activity before 28 weeks, but should avoid it after 28 weeks 9, 3
• After 28 weeks, limit to activities of daily living and low-intensity activity such as walking 9, 3
• Travel should be avoided after 28 weeks if requiring moderate-to-vigorous activity 9
• Any travel destination must have access to facilities capable of managing massive hemorrhage 9