Placental Cyst

A placental cyst is a fluid filled space in or on the placenta, found on ultrasound as a well defined anechoic structure with no flow on Doppler. Most are subchorionic or septal cysts caused by fibrin and cystic change in the chorionic villi. They matter because small cysts are harmless, while large or numerous ones can cause placental dysfunction and must be distinguished from tumours.

• Subchorionic cysts lie under the chorionic plate; intraplacental cysts lie within the villous tissue.
• Risk factors: older maternal age, multiparity, thickened placenta.
• Cysts above 4.5 cm may be linked with growth restriction.

Women have no symptoms. Large cysts may be linked with a small for dates uterus or polyhydramnios.

• Found on routine scan.
• Normal examination in most.
• Differential diagnosis: chorioangioma, subamniotic haematoma, placental lake, intervillous thrombus, mesenchymal dysplasia, hydropic villi of molar change.
• Complications: fetal growth restriction, compression of cord vessels, preterm birth, oligohydramnios, rarely stillbirth.

The approach is ultrasound with Doppler to confirm that the cyst is avascular.

• Ultrasound: round or oval anechoic space with a thin wall and no internal flow; measure in three planes.
• Colour Doppler to exclude a vascular tumour (chorioangioma has internal flow).
• Estimated fetal weight, amniotic fluid index, umbilical artery Doppler.
• Maternal glucose and blood pressure.

The principle is observation, with growth surveillance for large or multiple cysts.

• Supportive: reassure; normal activity; daily fetal movement counts; avoid smoking and alcohol.
• Pharmacological: none for isolated small cysts; aspirin 150 mg oral at night from 12 weeks to 36 weeks if risk factors for preeclampsia exist; labetalol 200 mg oral every 8 to 12 hours (maximum 2.4 g daily) for blood pressure 140/90 mmHg or more; oxytocin 10 IU IM at birth.
• Procedures: none for small cysts; growth scans every 4 weeks from 28 weeks for cysts above 4.5 cm; delivery at 39 to 40 weeks for isolated cysts, earlier for growth restriction or abnormal Doppler; send the placenta for histology.
• Red flags: reduced fetal movements, falling growth centile, oligohydramnios, rapid growth of the cyst, internal flow on Doppler.
• Follow-up: repeat scan at 28 and 32 weeks; antenatal review every 2 to 4 weeks.