Why Have my Breasts Shrunk After Having a Baby and an IUD?

After pregnancy, your body will slowly 'normalise' after the hormonal changes of pregnancy have abated. This effect will be prolonged and potentially exacerbated by breastfeeding. Breasts are made up of a mixture of glandular breast tissue and fatty tissue. The fatty component may change with your weight fluctuations and the glandular component will be subject to hormonal influences such as various different types of contraceptive pill and iucd. You should see a plastic surgeon for assessment of your breasts.

Unfortunately, what you are experiencing is what the large majority of women undergo after pregnancy.  It is a natural process called involutional ptosis that occurs after pregnancy and more so after breastfeeding.  It tends to get worse after each subsequent pregnancy.  It is a loss of breast volume and elasticity of the skin.  The solution is breast augmentation and sometimes breast lift.

This is an excellent question, and one very common for those seeking rejuvenation of the appearance of her breasts, especially after pregnancy.  Your complaints are likely due to post-pregnancy changes of your breasts as they likely enlarged and stretched during and immediately after your pregnancy as they became engorged with milk for lactation.  Now, they have a typical "deflated" appearance as they have settled but have lost much of the enlargement during your pregnancy.   

Options to ameliorate your complaints would include implant placement, breast lift, fat grafting, or a combination of these modalities.  A consultation with a plastic surgeon may help you decide which procedure(s) may be the best option for you.  

It may be wise to wait until after you have completed child bearing, as similar changes will occur after each pregnancy.  Also, it is not uncommon to have different sizes of breasts, but I would recommend evaluation if there is a sudden change in one side from another, a mass is felt to account for the size difference, any changes in skin or nipple appearance, or drainage/bleeding from the nipple.  Thank you for your question.  Hope that these answers help!