"I'm not pregnant on your show," Simpson, who previously appeared during both of her pregnancies, told DeGeneres.
"We got an IUD. Nothing's going to get in that uterus," she joked. "I have two beautiful children and I'm not having a third. They're too cute. You really can't top that."
Simpson and Johnson, who married in 2014, share 5-year-old daughter Maxwell and 3-year-old son Ace. The designer confirmed she hired mermaid performers and their "handlers" for Maxwell's birthday party in May.
"They had to be carried if they had to pee," she said of the women's restrictive silicon outfits. "They're literally, like, in the pool the whole time with the kids, trying to teach them to how to be mermaids. They had to be carried away [to the bathroom]."
Simpson had shared a family photo and dedicated a sweet message to Maxwell on her birthday May 1.
"My baby Maxwell is 5 years old today. She makes everyone's life mermaid magical," the star captioned the picture. "Happy Birthday to our fairytale princess. Mommy, Daddy, and Ace love you with our 'holy hearts' (as Max would say) #MerMax #MAXIDREW #canyoube5forever."