Is the ultrasound finding a placental lake or a placental cyst?

Distinguishing Placental Lakes from Placental Cysts on Ultrasound

Placental lakes and placental cysts are distinct entities with different locations, appearances, and clinical implications—lakes are intraplacental blood-filled spaces that appear as sonolucent areas within the placental parenchyma, while cysts are typically subchorionic or surface lesions located between the placenta and chorionic membrane.

Key Ultrasound Differentiating Features

Placental Lakes (Lacunae)

• Location: Within the placental parenchyma itself 1, 2
• Appearance: Homogenous sonolucent (anechoic) spaces, typically >2 cm in diameter 3
• Blood flow: Show turbulent lacunar blood flow on color Doppler imaging 1, 2
• Clinical context: Strongly associated with placenta accreta spectrum when multiple and irregular, especially with placenta previa and prior cesarean delivery 1, 2
• Pathophysiology: Represent blood-filled cavities with pulsatile, high-velocity flow from myometrium 2

Placental Cysts

• Location: Subchorionic (between placenta and chorionic membrane) or on the placental surface 4
• Appearance: Smooth-walled cystic structures protruding from the fetal surface 5, 4
• Blood flow: No internal blood flow on Doppler 4
• Pathology: Associated with subchorionic fibrin deposits with central cyst formation 4
• Clinical context: Often isolated findings, can be transient 6

Clinical Significance and Management

When Lakes Are Concerning:

Multiple large lakes (>4.5 cm) warrant increased surveillance 3, 4:

• Risk of fetal growth restriction increases significantly with lakes >4.5 cm (33% vs 0% for smaller lakes, p=0.01) 4
• Multiple lakes (>3) associated with higher risk of adverse outcomes 4
• When combined with thick placenta, risk of complications increases 7
• Critical distinction: In the context of placenta previa with prior cesarean delivery, multiple vascular lacunae are highly suggestive of placenta accreta spectrum (sensitivity 90.7%, specificity 96.9%) 1, 8

When Cysts Are Concerning:

• Cysts >4.5 cm or multiple (>3) associated with fetal growth restriction 4
• First-trimester placental cysts usually resolve spontaneously and have good prognosis when isolated 6
• Subchorionic cysts may indicate placental hypoxia when associated with abnormal pathology 5

Diagnostic Algorithm

1. Identify the location precisely:

   • Intraplacental = likely lake
   • Subchorionic/surface = likely cyst

2. Apply color Doppler:

   • Turbulent flow = confirms lake (especially concerning for accreta spectrum)
   • No flow = confirms cyst

3. Assess risk factors:

   • Prior cesarean + placenta previa + multiple lacunae = high suspicion for placenta accreta spectrum requiring expert evaluation at level III/IV center 1, 8
   • Isolated finding without risk factors = likely benign

4. Measure and count:

   • Document size and number

   • 4.5 cm or >3 lesions = increased surveillance for fetal growth 3, 4

5. Follow-up imaging:

   • Serial ultrasounds to monitor size changes
   • Cysts often resolve; lakes may evolve into thrombus 9

Critical Pitfall to Avoid

Do not dismiss multiple vascular lacunae in a patient with placenta previa and prior uterine surgery as benign "placental lakes"—this combination represents placenta accreta spectrum until proven otherwise and requires immediate referral to a specialist center with multidisciplinary expertise 1, 8. The sensitivity of ultrasound for detecting placenta accreta spectrum is 90.72% when performed by experienced providers 1.