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31 year old Karen Simpson, from Burscough, was last seen on Sunday morning when she left her home to walk to a local Tesco store.
She'd been suffering from post-natal depression.
Lancashire Police said a body, believed to be that of Mrs Simpson, was found in a field in her home village.
Mrs Simpson was married to husband Philip and had two sons and a three-month-old daughter.
She was seen leaving the Tesco store in Liverpool Road North at about 11am but did not return to her address in nearby Elm Road as expected.
In a statement, Lancashire Police said:
"Very sadly, at around 10.30am this morning, the body of a woman was found in a field off Marsh Moss Lane in Burscough.
"The body is believed to be that of missing 31-year-old Karen Simpson who had not been seen since the weekend.
"A post-mortem examination will be carried out to establish the cause of death but at this stage there are not thought to be any suspicious circumstances.
"We would like to thank all the community for their overwhelming support and assistance during the searches for Karen.''
Yesterday, her sister, Clair Dobrec, said Mrs Simpson had been receiving support for post-natal depression as the family made an emotional appeal for the young mother to return home.
A police spokeswoman said:
"Karen's family have been informed and are being supported by specially trained officers.''
The force said the death was not being treated as suspicious at this stage.
A post-mortem examination will be carried out to establish how she died.