Parker, who was previously publisher and sales director of Country Living and You & Your Wedding, is assuming responsibility for Prima, in addition to Country Living.
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Seaton, who was the publishing director for Prima, Prima Baby & Pregnancy, Babyexpert.com, Psychologies and NetDoctor.co.uk, is joining Immediate Media as the managing director of its London specialist division.
The newly created board-level post carries responsibility for all of Immediate Media's lifestyle brands, including parenting, outdoor and the Gardeners World magazine and website.
Seaton will be responsible for the magazine and website assets of You & Your Wedding and Prima Baby & Pregnancy.
Judith Secombe, group publishing director of Hearst Magazines UK's Lifestyle Group, said: "David's business development experience on Country Living will be a huge asset to Prima, as we continue to expand the brand and its content across new platforms.
"I'd also like to wish Steven the best of luck in his new role."
Parker said: "This is an exciting time for magazine publishers as we evolve our brands across new platforms, and I am delighted to be given the opportunity to build on and develop the success of Prima and Country Living."
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