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The singer and actress confirmed speculation that she is pregnant via a posting on her official website on Monday.
"It's true I'm going to be a mummy," she wrote, alongside a photo of herself dressed as a mummy for Halloween as she cradled her baby bump.
Simpson and Johnson were first rumoured to be expecting a baby last month after Simpson was spotted turning down a glass of champagne at Johnson's 32nd birthday party.
The pair will tie the knot in November in a lavish marriage ceremony on the Hawaiian island of Kauai.
Simpson and Johnson began dating in May 2010 and announced their engagement in November of the same year.
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