Pregnancy has turned pop star Jessica Simpson into a sex-crazed mama!

The "Irresistible" singer admits she can't get enough lovemaking with fiance Eric Johnson as the countdown to delivery begins.

The star, who was once called "sexual napalm" by ex-boyfriend John Mayer, says, "I'm kinda unstoppable at the moment."

And Simpson admits sex has never been better.

She tells Los Angeles DJ Ryan Seacrest, "The big O (orgasm) is, like, the biggest O ever."

Slideshow: Sexy Jessica Simpson Photos