What causes dense breasts?

Doctors are fairly sure that fluctuating estrogen levels are the culprit behind dense breasts, particularly since premenopausal women (who have higher levels of estrogen) are more likely to have dense breasts than postmenopausal women. Also, postmenopausal hormone therapy can increase the density of a woman's breasts.

Plus, it seems that breast density changes during the menstrual cycle, becoming denser in the latter part of the cycle when estrogen levels are highest. Thus, if you have dense breasts, it's a good idea to schedule your mammogram during the first two weeks of your cycle.