Changes in Hormones
Breasts often swell during periods in which hormone levels are changing, such as during pregnancy and just before your menstrual period. These hormone changes cause breasts to swell, making them appear larger in size. Breast-feeding and post-pregnancy can also cause breasts to swell with milk. However, breast size often returns to normal after hormone levels have gone back to normal and milk production stops.
In addition to swelling, pregnancy can cause breasts to change in other ways, like increasing nipple size and darkening the areolas.
Gaining Weight
Breasts are there to feed babies, so they contain milk ducts and glands. However, breasts are primarily composed of fat and fatty tissue. If you put on a few extra pounds, you can expect some of that excess weight to appear in the breasts. Filling out your jeans and skirts a little more also means that you may be filling out your bras and tops, too.
Medications
Medications that contain estrogen, like birth control pills or hormone replacement therapy, can lead to an increase in breast size. Estrogen can cause breasts to grow and swell, but stopping the medication can bring breasts back down to size.
