The 27-year-old reality TV star revealed the couple have already chosen a "controversial" baby name, and spoke of her fears her "alcohol abuse" might mean she could not have children.
Simpson also admitted she had nerves about the birth of her baby.
“I’m really nervous. I can’t even talk about it, I get so scared," she said. "I’ll make sure I’ve got some lip gloss and eyeliner. I don’t want to be a mess!"
She added: “Because I’d done Geordie Shore I’d convinced myself that I couldn’t have kids because of all the alcohol abuse."
The couple are expecting their first child on October 16.
In an interview with OK! magazine, she admitted they already have a name for their unborn child.
“It’s unusual and I’ve never heard anyone called it before. It can be used for a boy or a girl," she said.
"I think I’ll probably get some haters saying you shouldn’t call your baby that. It’s a bit controversial.”
Johnson, who rose to fame on The X Factor in 2014 as a part of boy bad Stereo Kicks, added: "I don’t think you’re ever ready until it happens, but we’re so excited. It feels amazing."
Simpson and Johnson have been dating for two and half years, after meeting on MTV show Single AF in 2017.