In a 32‑week pregnant woman with a posterior placenta measuring 5.5 cm thick and prominent lacunae, normal fetal growth and Doppler studies, and no change on 2‑week follow‑up, what is the most likely cause of these placental findings?

Placental Lacunae in a Posterior Placenta: Likely Placenta Accreta Spectrum

The prominent placental lacunae in this 32-week pregnancy with a posterior placenta most likely represent placenta accreta spectrum (PAS), even though the normal Doppler parameters and stable appearance over 2 weeks are somewhat reassuring. 1

Understanding Placental Lacunae

Pathophysiology of Lacunae in PAS

• Placental lacunae are sonolucent spaces within the placenta that result from exposure to high-velocity, pulsatile blood flow from the myometrium directly into placental tissue when normal decidualization fails. 1
• Multiple, large, and irregular lacunae are the most strongly associated ultrasound finding for placenta accreta spectrum, with the highest sensitivity and positive predictive value when identified in the second trimester. 1
• The presence of lacunae reflects abnormal trophoblast infiltration through a defective endometrial-myometrial interface, allowing maternal blood to pool in irregular spaces within the placenta. 1

Distinguishing PAS Lacunae from Benign Placental Lakes

• The key distinction is that PAS-associated lacunae demonstrate turbulent blood flow on color Doppler, occur with loss of the retroplacental clear zone, show myometrial thickness <1 mm, and are associated with placenta previa or low-lying placenta. 2
• Benign placental lakes are typically smaller, more uniform, and occur without the constellation of other PAS features. 2

Critical Diagnostic Challenge: Posterior Location

Why Posterior PAS Is Frequently Missed

• Posterior placenta accreta spectrum has dramatically lower ultrasound sensitivity (0-42.4%) compared to anterior PAS, with only 52.4-62% of posterior PAS cases detected prenatally. 3, 4, 5
• In one multicenter study, 70% of posterior PAS cases were not suspected antenatally because classic ultrasound signs (lacunae, hypervascularity, myometrial thinning, bridging vessels) were far less frequently visualized. 4
• Posterior PAS was discovered only at delivery in 46.7% of cases in systematic review, compared to much higher detection rates for anterior disease. 5

Why Normal Doppler May Be Falsely Reassuring

• The absence of hypervascularity at the bladder-wall interface is common in posterior PAS—none of the posterior cases in one systematic review showed this classic anterior PAS sign. 5
• Normal Doppler parameters do not exclude PAS; clinical risk factors remain equally important predictors even when imaging appears normal. 1, 6

Risk Factor Assessment Is Essential

Evaluate for These High-Risk Features

• Prior cesarean delivery combined with placenta previa or low-lying placenta (within 20 mm of the internal os) accounts for 49% of all PAS cases and >80% of confirmed accreta. 1, 6, 7
• The risk escalates from 3% with placenta previa alone to 11% with one prior cesarean, 40% with two, 61% with three, and 67% with five or more cesareans. 6, 7
• Prior uterine surgery with cavity entry (myomectomy, dilation and curettage) is present in 71-76% of posterior PAS cases and carries an odds ratio of approximately 2.8 for PAS. 7, 3, 5
• Advanced maternal age ≥35 years, multiparity (present in 82.5% of posterior PAS), Asherman syndrome, and in-vitro fertilization are additional independent risk factors. 6, 7, 5

Placental Thickness Consideration

• The 5.5 cm placental thickness is at the upper limit of normal for 32 weeks but is not a specific marker for PAS; however, increased thickness combined with grade 3 calcifications before 35 weeks would suggest placental dysfunction. 2

Recommended Management Algorithm

Immediate Actions

1. Obtain a detailed surgical history focusing on prior cesarean deliveries, myomectomy with cavity entry, curettage procedures, and any other uterine surgery. 6, 7
2. Perform a meticulous re-evaluation of the placental location relative to the internal cervical os—placenta previa is present in 92.8% of posterior PAS cases. 3, 5
3. Reassess for additional ultrasound markers that may have been subtle:
   • Loss of the normal hypoechoic retroplacental clear zone (present in 41.1% of posterior PAS) 5
   • Myometrial thickness <1 mm (present in 4.4-70% depending on detection) 4, 5
   • Bladder-wall irregularity (present in 16.6% of posterior PAS) 5
   • Placental bulge or extension into/beyond the myometrium 1

If Risk Factors Are Present

• Even with normal Doppler and stable appearance, the combination of prominent lacunae + posterior location + any prior uterine surgery mandates preparation for possible PAS at delivery. 6, 4, 5
• Refer immediately to a Level III or IV maternal care facility with a multidisciplinary team including maternal-fetal medicine, experienced pelvic surgeons (gynecologic oncologists for suspected percreta), urologic surgeons, interventional radiologists, obstetric anesthesiologists, and a blood bank with massive-transfusion protocols. 1, 6
• Plan cesarean delivery at 34+0 to 35+6 weeks gestation if PAS is suspected, because approximately 50% of women who remain pregnant beyond 36 weeks require emergent delivery for hemorrhage. 6

Surgical Preparation

• The standard operative approach for confirmed PAS is cesarean hysterectomy with the placenta left in situ after fetal delivery; manual removal of the placenta is absolutely contraindicated because it precipitates catastrophic hemorrhage. 1, 6
• Activate a massive transfusion protocol with packed red blood cells, fresh-frozen plasma, and platelets in a 1:1:1 to 1:2:4 ratio if hemorrhage occurs. 6

Key Clinical Pitfalls to Avoid

• Do not be falsely reassured by normal Doppler or stable 2-week follow-up—posterior PAS has poor ultrasound sensitivity and may appear deceptively benign. 4, 5
• Do not rely solely on imaging—the absence of ultrasound findings does not exclude PAS when clinical risk factors are present. 1, 6
• Do not assume benign placental lakes—prominent lacunae in the setting of any prior uterine surgery or posterior low-lying placenta warrant high suspicion for PAS. 2, 5
• Do not attempt manual placental removal at delivery if the placenta does not separate spontaneously—this is the most dangerous error and requires immediate multidisciplinary team activation. 1, 6

Prognostic Context

• Posterior PAS has a lower rate of hysterectomy (48%) compared to anterior PAS (89%) and a higher rate of successful conservative management with balloon tamponade (52% vs. 17%), but this does not reduce the need for expert surgical planning. 3
• When posterior PAS is diagnosed antenatally and managed at a referral center, outcomes are significantly better than when discovered unexpectedly at delivery. 1, 6