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Placental cysts are sonolucent areas that can be detected antenatally by ultrasound; they have a round or oval cavity, usually isolated from the placental circulation and contain a gelatinous fluid. These cysts are located within the placental tissue (septal cysts), or under the fetal plate (subchorionic cyst
Most placental surface cysts are associated with a normal pregnancy outcome. Most such cysts are related to cystic change in an area of subchorionic fibrin. Cysts larger than 4.5 cm or more than 3 in number are more frequently associated with intrauterine growth restriction.
Subchorionic placental cysts are more serious. When attached near the umbilical cord
insertion, the risk of umbilical cord constriction is increased. This may cause fetal growth retardation and intrauterine asphyxia.
Usually no alteration of the obstetrical management is indicated.There is no likelihood of cyst rupture unless its massive so complete pelvic rest is not advised as such for placental cyst alone.
Since you are not aware of the size or location you may avoid strenous activity till further follow up however complete pelvic rest is not mandatory.
More important is to rule out any associated conditions which can be present simultaneously and your doctor might need to rule them out if suspected.
Some of the conditions associated with evidence of placental cyst are as follows,
Common associations: Placental cysts occur frequently in Diabetes Mellitus
or maternal fetal rhesus incompatibility.
Uncommon associations:Beckwith –Wiedemann syndrome,Hemochromatosis
,Intrauterine growth retardation
,McCune –Albright syndrome
,Protein C deficiency,Zellweger syndrome; partial molar pregnancy and fetal rubella.
Please do not panic as majority of pregnancies with placental cysts are normal and such associations or complications are rare and only need to be tested if the regular fetal monitoring indicates any other suspicious findings.
As of now you need not worry and consider your pregnancy normal unless further investigations reveal any risk.
Wish you and your baby good health forever.