There is a difference between breast swelling during pregnancy and breast swelling before menstruation. Breast swelling during pregnancy is caused by changes in hormone levels. After pregnancy, breast tissue will develop rapidly, mainly in preparation for breastfeeding, and the size of the breasts will also increase significantly compared to the non-pregnant period. The breast swelling before menstruation is usually caused by breast enlargement, the size of the breast will not change, and the swelling and pain will be relieved after menstruation. For women who have been experiencing breast swelling and pain before menstruation, it is recommended to have an ultrasound of the breast to see if there is breast enlargement, fibroids, etc.