Excerpt from Strong Women, Strong Bones: Everything you need to know to prevent, treat, and beat osteoporosis.

Unless we take vitamin D supplements, our bones change with the seasons, reflecting our exposure to the vitamin D-forming rays of the sun. Bone density reaches its annual peak after the summer. Seasonal losses begin later in the fall as days get shorter. We reach our annual low in late spring. And then the days grow longer again. The difference between high and low points can amount to 2 to 4 percent. That's why it's very important to consistently schedule bone density tests for the same time of year - otherwise you may just see seasonal changes.