If your breasts are growing, it’s likely due to weight gain (because breasts are made up of mostly fat cells, in addition to glandular tissue), pregnancy (as milk ducts ramp up for breast-feeding), birth control pills, and/or hormonal changes related to your menstrual cycle (they’re very sensitive to shifts in estrogen and progesterone). No need to worry, though you might need to buy new bras.

If you find your chest shrinking, you’ve likely either lost weight or your estrogen levels are falling, perhaps from going off the pill or impending menopause. If you’re seeing shrinkage and none of those apply (especially if you also have hair loss, acne, and facial hair), consider being tested for polycystic ovary syndrome (PCOS), which is characterized by high levels of the hormones testosterone and DHEA. Or maybe just lay off the Starbucks. A study in the British Journal of Cancer found that drinking three cups of coffee a day caused breasts to shrink in some women, with additional cups increasing the effect. The culprit is a gene shown to link breast size to coffee consumption.