Ashlee Simpson may have put to rest speculation she is pregnant by showing off her slim physique in a bikini on the cover of a new magazine.

After Simpson and rocker beau Pete Wentz announced their engagement earlier this month, sources immediately alleged the Boyfriend singer is expecting the couple's first child.

But, posing on the June edition of U.S. magazine Shape in a red string bikini, the 23-year-old proudly shows off her newly fit figure - with her stomach looking as toned and flat as ever.

She says, "Now I'm in shape. I never thought I'd be on the cover of a magazine in a bikini but now is the time and place to show the world how I'm feeling about myself."