Jo Dee Messina is PREGNANT!

Country singer Jo Dee Messina announced yesterday that she is pregnant--and in her third trimester!

Messina, 38, just celebrated her one-year anniversary with Chris Deffenbaugh, 28, last week.

"We knew we wanted to have kids, but we just said, 'Whenever it happens, it happens.' And it happened!" she says. "We were both surprised at first. It's such a lifestyle change. But then we're like, 'Wow, we're going to be parents!'" she told People Magazine last week.

Messina and Deffenbaugh are expecting a boy sometime in December.