Hi EnMich13! Your concerns stem from the fact that the breasts are highly specialized endocrine glands that significantly respond to fluctuations in weight and are very sensitive to hormonal changes, especially during puberty and pregnancy. With weight loss or gain, usually all areas of the body respond proportionally to this change, although some women genetically tend to lose or gain more mass in certain areas of their bodies, whether it is their hips, love handles, or breasts. Your exercise and weight loss, although healthier for your heart and body, can lead to a significant reduction in your breast size. In addition, substantial fluctuations in hormone levels during puberty and pregnancy can lead to rapid growth of the breast tissue and ducts, sometimes asymmetrically, leading to a condition called juvenile breast hypertrophy or gestational breast hypertrophy, respectively. The usual culprits are a large surge of prolactin, estrogen, and progesterone levels during these events for which some women's breasts are more sensitive to these changes than others due to increased hormone receptors or genetic factors. After puberty or pregnancy is completed, hormone levels usually return back to normal levels, which leads to breast volume deflation with reduction in size and occasionally ptosis (sagging). My guess based on your description is the significant changes in your hormone levels from completing puberty and pregnancy, followed by further weight loss from exercise has led to the loss of volume and firmness of your breasts. I am not aware of IUDs by themselves leading to loss of breast size, although the cessation of monthly periods from use of some IUDs can negatively affect your monthly hormonal cycles, which can secondarily affect your breast volume. The best way to regain your breast size, short of gaining weight or becoming pregnant again, would be to consult your local board-certified plastic surgeon for breast augmentation with implants +/- breast lift. Although fat stem cell transfer research looks promising, it is still experimental at this time and would only lead to a modest increase in volume. Hope that helps!- Dr.92660.