The 39-year-old, who signed up to launch the ITV show in late April, received cheers from her co-hosts as she made her announcement.
Hawkins - previously a familiar face on Sky News's Sunrise programme - told viewers after the programme featured the story of a woman whose waist had grown to 55ins during her first pregnancy.
She responded: "I'm pregnant, I'm happy to announce, expecting a baby - and that kind of story fills me with dread.
"I'm hoping I won't reach that record."
The presenting team already have seven children between them - Susanna Reid has three sons, Ben Shephard has two sons, and Sean Fletcher has a son and a daughter.
Reid joked: "Who would have thought that working on Good Morning Britain could be so productive!"
And Fletcher said: "That explains why you've been eating chocolates all morning every morning, and you force them on me as well."
Hawkins - married to husband Mark Herbert for six years - said later: "My husband Mark and I are absolutely thrilled to be expecting our first child. I've been looking forward to sharing the news with everyone, and can't wait to be a mother for the first time."