Jamie Lynn Spears went under the knife thinking she was fat when really she was pregnant, according to US reports.

The pop princess' sister reportedly begged her mother for liposuction when she fell pregnant because she thought she had simply eaten too many cheeseburgers.

A source tells Star, "Jamie Lynn had just finished her hit Nickelodeon show Zoey 101 when she found the weight piling on.

"Not realizing she was expecting, she pleaded with her mom, Lynne Spears, to let her get liposuction."

The source adds, "She didn't know she was pregnant when she filled out the health questionnaire prior to the procedure.

"Her mom approved the injections and went through tons of red tape to get the clinic to administer them to an underage patient."

See Jamie Lynn through the years below