Your breasts are made up of connective tissues that include a system of tiny passages that carry milk to the nipples (milk ducts). Mammary duct ectasia occurs when a milk duct beneath the nipple widens. The duct walls may thicken and fill with fluid, becoming blocked or clogged with a sticky substance. Inflammation may result. Experts don't know exactly what causes mammary duct ectasia. Some speculate the cause to be associated with: Breast tissue changes due to aging. As you age, the composition of your breast tissue changes from mostly glandular to mostly fatty in a process called involution. These normal breast changes can sometimes lead to a blocked milk duct and the inflammation associated with mammary duct ectasia. Smoking. Cigarette smoking may be associated with widening of milk ducts, which can lead to inflammation and, possibly, mammary duct ectasia. Nipple inversion. A newly inverted nipple may obstruct milk ducts, causing inflammation and infection. A nipple that's newly inverted could also be a sign of a more serious underlying condition, such as cancer.