"I'm not pregnant," she announced Monday on "The Ellen DeGeneres Show." "We got an IUD. Nothing’s gonna get in that uterus."
The singer also explained that she's satisfied with the two children she already has -- 5-year-old Maxwell and 3-year-old Ace.
"I have two beautiful children and I’m not having a third," Simpson, 36, said. "They’re too cute ... you can't top that."
Simpson and her husband Johnson, 37, have been together seven years.
"I don’t know if I’ve ever had a seven-year relationship other than with a woman," she said, later clarifying she meant friendships with women.
Simpson said there's a secret to the success of her and Johnson's marriage: nightly massages.
"Even if things aren’t going great, and he’s still snoring, somehow he knows how to massage my back," she said.