Bristol Palin is coming clean, speaking out for the first time about all the buzz over her new face.

Startling before-and-after photos in the new issue of Us Weekly highlight Bristol's dramatically different jaw line.

In a layout titled, "Why I Did It," Bristol says, "I had corrective jaw surgery. Yes, it improved the way I look, but this surgery was necessary for medical reasons."

She insists: "It's not plastic surgery."

Sarah Palin's eldest daughter says ever since she was a child growing up in Alaska she knew she would, at some point, have to have the surgery  

"My dentist warned me I'd have to have surgery when I was old enough because my jaw wasn't aligned. My mom knew I was going to have this one day […] I wouldn't get plastic surgery unless I got in an accident or something terrible and got disfigured! I don't obsess over my face."

But she adds, "I am absolutely thrilled with the results. I look older, more mature, and don't have as much of a chubby little baby face!"