The 33-year-old shared her first "bump" snap on Instagram, joking that she didn't have to try and hide it anymore.
"Beyond excited to finally be able to tell you that @pdoranjones and I are expecting our first little one together, due in September," she wrote.
"We are both completely over the moon! Now I can stop trying to hide the bump. Thank you for all your lovely messages so far #bumptastic."
Hardman, who revealed her pregnancy news exclusively to Hello!, said she's "never felt more beautiful".
"I'm watching my body change daily, which is weird and wonderful," she said.
"I'm not thinking about the birth, I'm not thinking about getting the nursery ready, I'm literally just thinking, 'Today is such a magical day'."
This will be Hardman's first child. Doran-Jones is already dad to three-year-old Isla from a previous relationship.
The TV presenter previously confessed she developed anorexia and had battled with the disease for four years in her 20s. She was told she "might not" be able to have children.
Hardman is now an ambassador for eating disorder charity Beat.
Read the full interview in the latest issue of Hello! magazine, out in stores from 25 April.