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It appears to be case of Pseudopancreating cyst
.It requires major operation .You have not mentioned about duration of pregnancy
& time since she is having this cyst .where the cyst is causing any problem or silent cyst.So treatment modality depends on many factors.If it is silent or asymptomatic ,plan treatment after birth of baby.So consult some good surgeon to plan treatment depending upon your health & duration of pregnancy.
Wish you good health