Pregnancy affects levels of estrogen and progesterone in your body. These hormones get the breasts ready for lactation and are responsible the breast changes you may experience. Estrogen stimulates growth of the breast duct cells and generates the secretion of prolactin, the milk making hormone. Prolactin stimulates breast enlargement and milk production. Progesterone supports the formation and growth of milk-producing cells within the glands of the breasts. In the 2nd trimester, estrogen levels continue to rise. Milk ducts continue to develop, making the breasts feel heavy or full. Some need to purchase a larger bra at this time as you can increase one cup size or several. A few days after birth when milk transitions from colostrum to mature milk, breasts can engorge and grow even more. For several weeks, breasts will feel full between feedings and soft after feeding. Eventually they will feel soft all the time and start to shrink back to pre-pregnancy size. Some breasts will stay large and some will shrink back significantly. Sometimes one can reduce in size and the other stay large.
I usually recommend not purchasing a bunch of bras while pregnant. You don’t know what size you’ll end up being and may waste money on expensive bras you won’t wear. Bralettes, nursing tanks with built in shelf bras, or soft wireless bras that have a lot of stretch can be helpful while your body is constantly changing. Once you’re several months into nursing, breasts may stabilize in size until you completely wean. Now it’s time to be re-measured for the support your girls deserve.