The Big Brother star announced that she had fallen pregnant despite being told by doctors that she had a "very, very low chance" of conceiving naturally after bouts of extreme dieting.
"I'm going to be the yummiest mummy there is," she told OK! Magazine. "I'm milking it, trust me. I've told my friends and family, if I'm going through it, you're all going through it.
"Alex is desperate to know if it's a boy or a girl but I can't tell him until the four-month scan. We want to know the baby's sex."
Houghton revealed that the couple discovered the news four weeks ago, mere days before they were due to start IVF treatment.
"I couldn't move. The only thing that would move was my hands, shaking. I was in shock," she said. "We were a week away from starting IVF. The thought of conceiving naturally had gone out of my mind completely - I didn't even think it was possible."
Houghton added: "I'm having more scans than I'd usually have - not because I need to, just because I want to for my own peace of mind. I don't care how much it costs me, I just want to know everything is alright. I'm just frightened. I want the nine months to be over and for everything to be okay."
Houghton and Reid became engaged in October after dating for little less than five months.