NEW YORK — Lifetime has signed Kirsten Dunst (“Little Women,” “Interview With the Vampire”) to star in an original movie, “Fifteen and Pregnant: A Nice Girl’s Story.”Director Sam Pillsbury (“Free Willy 3: The Rescue”) will start filming Aug. 26 in Portland, Ore. Based on a true story, “Fifteen” is about an unmarried middle-class teenager whose pregnancy ends up alienating her boyfriend, who walks out on her; her parents, who think she’s not taking her responsibilities seriously enough; and her younger brother and sister. Park Overall and David Andrews play the parents. Diana Kerew is executive producer for Hearst Entertainment. Susan Cuscuna is the writer. Lifetime will schedule pic in January 1998.