What Are Placental Lakes?
Placental lakes are sonolucent or hypoechoic areas visualized on ultrasound within the placenta, representing dilated intervillous spaces filled with maternal venous blood. 1, 2
Ultrasound Appearance and Characteristics
Placental lakes appear as anechoic (echo-free) or hypoechoic zones within the placental tissue on grayscale ultrasound imaging. 1 These structures have been given multiple names in the literature including "placental venous lakes," "placental lacunae," or "placental caverns." 1
Key Imaging Features:
- Sonolucent spaces within the placental parenchyma that represent areas where villous structures are absent 3
- Maternal blood accumulation in the intervillous space 3
- May show turbulent blood flow on color Doppler examination 2, 3
- Can demonstrate dynamic changes with uterine contractions (reducing in size during Braxton Hicks contractions and enlarging afterward) 3
Clinical Significance and Differential Diagnosis
Benign vs. Pathological Lakes
Most placental lakes are physiological findings representing normal dilation of the intervillous space and typically have a good obstetrical outcome. 2 However, the clinical significance varies considerably based on size, location, and associated findings. 1
Important Distinction from Placenta Accreta Spectrum
Critical caveat: Placental lakes must be distinguished from vascular lacunae associated with placenta accreta spectrum (PAS), which represent a completely different and dangerous pathology. 4
- Multiple vascular lacunae with turbulent lacunar blood flow on Doppler are the most common and sensitive finding for placenta accreta spectrum 4, 5
- In PAS, these represent massively dilated uterine vessels (not placental vessels) at the uteroplacental interface 4
- PAS-associated lacunae occur with loss of the retroplacental clear zone, thinned myometrium (<1mm), and placenta previa 4, 5
When Placental Lakes May Signal Pathology
Placental lakes warrant increased surveillance when associated with: 1, 6
- Fetal growth restriction (FGR) - lakes are frequently observed in FGR/SGA pregnancies, especially in the second trimester 7, 6
- Unusually large size - lakes larger than the total placental area or occupying the entire retroplacental space 2
- Subchorionic location - particularly when associated with abnormal fetal Doppler findings 7
- Arterial-type blood flow on Doppler examination (suggests abnormal vascular communication) 2
Diagnostic Approach
Ultrasound Evaluation
When placental lakes are identified, the sonographer should: 1, 6
- Document size, number, and location of the lakes 1
- Apply color Doppler to characterize blood flow patterns (venous vs. arterial, turbulent vs. laminar) 2, 3
- Assess for PAS features if anterior placenta or risk factors present (prior cesarean, placenta previa, loss of retroplacental clear zone, myometrial thinning) 4, 5
- Evaluate placental thickness and calcifications - increased thickness and grade 3 calcifications before 35 weeks suggest placental dysfunction 6
- Perform fetal biometry and Doppler studies to assess for growth restriction and placental insufficiency 7, 6
Multidisciplinary Collaboration
Diagnosis requires close collaboration between sonographers, obstetricians, geneticists, and pathologists when lakes are associated with concerning features, as some etiologies may be oncological, fetal, or require multidisciplinary surgical management. 1
Management Implications
- Benign placental lakes typically require no specific intervention beyond routine prenatal care 2
- Large or multiple lakes with FGR warrant increased fetal surveillance with serial growth scans and Doppler studies 7, 6
- Lakes suspicious for PAS require referral to a level III/IV maternal care facility with multidisciplinary expertise for delivery planning 4
- Awareness of potential delivery complications - even benign-appearing large lakes can occasionally cause obstetric hemorrhage during second stage of labor 3