FRIDAY, May 27 (HealthDay News) -- People continue to grow well into their 70s, but they're not growing taller, they're growing wider, new research shows.

Researchers at the University of North Carolina at Chapel Hill School of Medicine found that people's pelvic bones (hip bones) continue to grow long after they reach skeletal maturity. The end result: extra inches at the waistline and many extra pounds on the scale.

"Whether or not they also have an increase in body fat, our findings suggest that pelvic growth may contribute to people becoming wider and having a large waist size as they get older," said Dr. Laurence E. Dahners, senior study author and a professor of orthopedics at UNC, in a university news release. "If the rest of the body is widening, this might account for a significant portion of an increase in body weight of about one pound a year."

In conducting the study, published online this week in the Journal of Orthopaedic Research, researchers used CT scans to measure the width and height of 246 patients in various age groups. The study found the width of the patients' pelvises continued to grow, even after skeletal maturity at 20 years of age. Specifically, the researchers noted, the pelvic inlet (the birth canal opening in the middle of the pelvis) widened, which is evidence of actual pelvic growth.

On average, the pelvic width of the oldest patients in the study was nearly an inch larger than the width of the youngest patients. The study's authors explained this one-inch increase alone could amount to three extra inches on the waist between the ages of 20 and 79 years. They also noted this widening could also account for a significant portion of an annual one-pound weight gain.

More information

The U.S. National Institutes of Health provides more information on how aging affects bones.