Jo Dee Messina is expecting her first child. The country singer announced Tuesday that she's seven months pregnant with a baby boy.

"We knew we wanted to have kids, but we just said, 'Whenever it happens, it happens.' And it happened," the 38-year-old singer tells PEOPLE magazine.

Messina discovered the news after being frustrated that her diet wasn't resulting in as much weight loss as she'd hoped. She says she was excited to realize the extra weight was "all baby!"

"We were both surprised at first. It's such a lifestyle change," she says. "But then we're like, 'Wow, we're going to be parents!'"

But Messina doesn't plan to change her exercise routine much. The fitness buff plans to run a five-mile race on Thanksgiving Day.

Messina and husband Chris Deffenbaugh celebrated their one year wedding anniversary last week.