A 22-week ultrasound shows a fetal anomaly with a cystic mass in the lower abdomen displacing bowel loops. Which ultrasound feature best distinguishes an ovarian cyst from a meconium pseudocyst in a female fetus?

## Distinguishing Fetal Ovarian Cyst from Meconium Pseudocyst ### Definition and Embryologic Origin **Fetal ovarian cysts** are benign, fluid-filled structures arising from the ovary, typically located in the pelvis anterior to the bladder. **Meconium pseudocysts** (or meconium peritonitis cysts) result from in-utero bowel perforation with spillage of meconium into the peritoneal cavity, forming loculated collections that may mimic cysts. ### Key Discriminating Feature **Key Point:** The **location and character of the cyst** is the best discriminator. Ovarian cysts are typically **anterior to the bladder, unilocular, with clear anechoic fluid and thin walls**. Meconium pseudocysts are **intra-abdominal, often multiloculated, with internal echoes and debris**, and are frequently associated with calcifications. ### Comparative Ultrasound Findings | Feature | Fetal Ovarian Cyst | Meconium Pseudocyst | | --- | --- | --- | | **Location** | Pelvis, anterior to bladder | Intra-abdominal, variable | | **Fluid character** | Clear, anechoic | Echogenic, debris-filled | | **Wall appearance** | Thin, smooth | Thick, irregular; may calcify | | **Internal echoes** | Absent (simple cyst) | Present (meconium, blood) | | **Multiloculation** | Rare (usually unilocular) | Common | | **Associated findings** | None; isolated | Bowel perforation, ascites, calcifications | | **Prognosis** | Excellent; usually regress | Depends on extent; may require postnatal surgery | ### Clinical Pearl **Clinical Pearl:** Fetal ovarian cysts are a common incidental finding (1–2% of pregnancies) and are almost always benign and self-limited, requiring only follow-up ultrasound. Meconium pseudocysts, by contrast, indicate in-utero bowel perforation and warrant detailed fetal assessment and postnatal planning. ### High-Yield Mnemonic **Mnemonic: "OVARY" = Ovarian location, Vertical pelvis position, Anechoic/clear fluid, Regresses spontaneously, Young age (fetal)** **Mnemonic: "MECONIUM" = Multiloculated, Echogenic debris, Calcifications, Originating from bowel perforation, Needs postnatal imaging** ### Tip **Tip:** Always trace the cyst back to its organ of origin. If you can identify the ovary as the source and the cyst is simple (clear fluid, thin wall, anterior pelvic location), it is almost certainly an ovarian cyst. If the cyst is intra-abdominal, echogenic, and associated with ascites or calcifications, suspect meconium pseudocyst and look for signs of bowel perforation (hyperechoic bowel loops, intra-abdominal calcifications). [cite:Callen's Ultrasonography in Obstetrics and Gynecology 6e Ch 13]