In a normal pregnancy without uterine anomalies or prior surgery, how does placental migration occur and what ultrasound follow‑up and management are recommended for a low‑lying placenta?

Placental Migration During Pregnancy

Mechanism of Placental Migration

Placental "migration" is not true anatomical movement but rather differential growth of the lower uterine segment, which causes the placenta to appear to move away from the internal cervical os as pregnancy advances. 1

The phenomenon occurs through:

• Trophotropism: Preferential growth of placental tissue toward areas with better blood supply (typically the fundus), while the portion near the poorly vascularized lower segment atrophies 2
• Dynamic development of the lower uterine segment: As the lower uterus stretches and elongates in the third trimester, the relative position of the placental edge changes 3

Natural History and Resolution Rates

The vast majority (90-95%) of low-lying placentas identified in the second trimester will resolve by the third trimester. 2, 3

Key statistics on resolution:

• 25-42% of pregnancies show low-lying placenta at 11-14 weeks, dropping to 3.9% at 20-24 weeks and only 0.4% at term 4
• 92.4% of second-trimester low-lying placentas migrate to the upper segment by the third trimester 2
• Migration occurs in 38% of cases initially diagnosed between 28-32 weeks when followed to 36 weeks 5

Factors Affecting Migration Likelihood

Distance from Internal Os

The initial distance between the placental edge and internal cervical os is the strongest predictor of whether migration will occur:

• When the distance is >2 cm: 92.4% migration rate 2
• When the distance is <2 cm: 68% migration rate 2
• When the distance is <1.5 cm: migration rarely occurs 2
• Complete placenta previa (covering the os): migration is unlikely, especially beyond 28 weeks 5

Placental Location

Anterior placentas migrate more reliably than posterior placentas:

• Anterior placenta: 94.5% migration rate 2
• Posterior placenta: 90.2% migration rate 2
• Posterior placenta previa within 1 cm of the internal os does not migrate during the third trimester 5

Prior Uterine Surgery

Previous cesarean delivery significantly reduces migration rates:

• No prior cesarean: 95.1% migration 2
• Prior cesarean delivery: 77.7% migration 2
• Prior D&C or manual removal of placenta: 55.5% migration 2
• Women with prior cesarean have 3.7-fold increased risk of persistent low-lying placenta at term 6

Timing Considerations

Placental migration does not occur beyond 36 weeks' gestation 5

Ultrasound Follow-Up Protocol

Initial Assessment (18-22 Weeks)

Transvaginal ultrasound is the gold standard for accurate diagnosis, as transabdominal ultrasound changes the diagnosis in 26% of cases when landmarks are suboptimally visualized. 1, 7

Initial evaluation should include:

• Transabdominal ultrasound first, followed by transvaginal ultrasound for precise measurement of the internal os distance (IOD) 1, 7
• Color Doppler to identify vasa previa and assess placental blood flow 1
• Documentation of placental location (anterior vs. posterior) and exact distance from internal os 1

Follow-Up Timing Based on Initial Findings

If placental edge is ≥2 cm from the internal os at 18-20 weeks: no follow-up ultrasound is necessary 1

If placental edge is <2 cm from the internal os or covering it: follow-up ultrasound at 28-32 weeks is essential 1, 5

For persistent low-lying placenta at 28-32 weeks: repeat ultrasound at 32-36 weeks to finalize delivery planning 1, 5

Revised Cut-Off for Anterior Placentas

For anterior low-lying placentas, the follow-up threshold can be lowered from 20 mm to 5 mm without missing high-risk cases, reducing unnecessary follow-up scans. 6

This is because anterior placentas require an IOD cut-off of only -4.5 mm (meaning 4.5 mm overlap) to identify all cases that will persist as abnormally located in the third trimester, compared to 15.5 mm for posterior placentas 6

Management Recommendations

Activity Restrictions

Before 28 weeks: women with asymptomatic low-lying placenta can continue moderate-to-vigorous physical activity 4

After 28 weeks: avoid moderate-to-vigorous activity but maintain activities of daily living and low-intensity walking 4

Screening for Placenta Accreta Spectrum

All women with persistent low-lying placenta or placenta previa who have prior cesarean delivery must be evaluated for placenta accreta spectrum disorder, as the risk increases 7-fold after one cesarean and up to 56-fold after three cesareans. 1, 4

Ultrasound features to assess:

• Intraplacental lacunae (highest sensitivity and positive predictive value for accreta) 1, 8
• Loss of retroplacental clear zone 1, 8
• Myometrial thickness <1 mm 1, 8
• Abnormal Doppler flow patterns with turbulent lacunar blood flow 8

Delivery Planning for Persistent Cases

If the placenta remains within 2 cm of the internal os at 32-36 weeks, plan cesarean delivery at 34 0/7 to 35 6/7 weeks at a facility with blood bank capabilities and multidisciplinary expertise. 4

Delivery should not be delayed beyond 36 weeks, as approximately 50% of women with placenta accreta spectrum beyond 36 weeks require emergent delivery for hemorrhage. 4

Critical Safety Precautions

Digital pelvic examination must be avoided until placenta previa has been excluded by ultrasound, as manipulation can trigger catastrophic hemorrhage. 1, 4

Notify the blood bank in advance of planned delivery due to frequent need for large-volume transfusion. 1, 4

Optimize maternal hemoglobin during pregnancy by treating anemia with oral or intravenous iron supplementation. 4