Osteoporosis, or bone thinning—a condition that can lead to serious spine or hip fractures—affects more than 10 million people in the U.S., most of them women, according to the National Osteoporosis Foundation.

But what about those who don't quite hit the diagnostic mark for full-blown osteoporosis? That's the case for some 43 million Americans, who have been told they have osteopenia—bone density that's lower than normal.

To diagnose osteoporosis you need to undergo a bone density test. You have the condition if that test shows your "T-score"—which is determined by comparing your bone density with that of a healthy young adult of your gender—is less than -2.5, according to the National Institutes of Health. You have osteopenia if your T-score is between -1.0 and -2.5.

You’re more likely to have osteopenia if you’re a postmenopausal woman, you’ve broken a bone, you have rheumatoid arthritis, you smoke or consume a lot of alcohol, or used corticosteroid drugs like prednisone for at least three months.

Will You Develop Osteoporosis?

Having osteopenia does put you at an increased risk for osteoporosis. But most of the time, osteopenia doesn’t progress to osteoporosis.

Less than 10 percent of postmenopausal women with mild osteopenia develop the condition within 15 years, less than 10 percent of women with moderate osteopenia develop it within five years, and less than 10 percent of women with advanced osteopenia progress to osteoporosis within one year, according to a 2012 study in the New England Journal of Medicine.

Most of the time, no treatment for osteopenia is needed other than diet and lifestyle changes. The evidence that osteoporosis drugs like alendronate (Fosamax and generic) help during this stage is inconsistent. Instead, consider a repeat bone scan after several years. (In some cases, such as when older adults have already had fall-related fractures, those with osteopenia may benefit from medication.) An online fracture risk-assessment tool called FRAX may help your physician assess whether or not you need medication. (But note that risk calculators have potential shortcomings.)

Bone-Saving Strategies

Our medical experts recommend a baseline bone-density scan (known as a DEXA scan) at age 65 for women (and earlier if you have risk factors such as those above). Men who are age 70 and older may want to talk with a doctor about the risks and benefits before deciding whether to be screened. (Read more about when you need to get your bone density measured, and when you don't.)

“In general, I think it’s a good idea for women with any risk factors to have a bone-density test as they go through menopause,” says Felicia Cosman, M.D., senior clinical director at the National Osteoporosis Foundation and a professor of medicine at the Columbia University Medical Center in New York City.

If your scan reveals osteopenia, lifestyle measures may help slow bone loss. They include 30 minutes of weight-bearing exercise, such as walking, on most days of the week, and for most women who have gone through menopause, making sure to get 800 daily IU of vitamin D and consuming a diet that’s rich in calcium.