First of all, the latest statistic I read is that about 80% of women have some degree of Fibrocystic Breast Disorder (it is not a "disease"). Secondly, FCBD is not linked with breast cancer. While breast augmentation is not contraindicated in women with FCBD, there are a couple of factors you should consider. You did not mention if the reason you get an annual breast ultrasound is if you have palpable masses in your breasts. If a woman has a breast mass on exam, it is the burden of the physician to rule out cancer. Usually a mammogram is performed, but it may not be diagnostic unless "suspicious". In addition, very youthful breasts may not be the best way to image the breasts since they tend to be very "dense" on a mammogram. So an ultrasound is helpful to image dense breasts, and it particularly helpful to determine if a mass is"cystic" or "solid". If it is clearly a simple cyst, nothing has to be done, but often a needle aspiration is performed to remove fluid to minimize the size or symptoms of tenderness. A solid mass can be benign (fibroadenoma, more common in young women, not imperative to remove, but can grow to be large, so often removed in the operating room), but because of the possibility of something suspicious, it is usually biopsied, many times by way of a relatively non-invasive ultrasound guided needle aspiration procedure under local anesthesia.
What does this mean to you.....well, in part, it is related to your age. You didn't mention how old you are, but since you are having regular ultrasounds, presumably you are "young" and your breasts are "dense. You didn't mention that you have had any biopsies, so presumably all the masses in your breasts have been "simple cysts". You didn't mention your breast cancer risk, but unless you are at "high risk" (positive genetic testing or strong family history despite negative genetic testing), breast augmentation should not be a contraindication. Some plastic surgeons will tell you that saline implants under the muscle will be the ideal option for mammogram imaging, but I have discussed this with top-notch radiologists who inform me that as long as the implants are soft and mobile without capsule contracture, that it probably doesn't make any difference as long as the radiology tech is very good at the Eklund technique...the tech compresses the implants away from the breasts in order to get the most breast tissue on the xray cassette.
So your answer about FCBD is somewhat based on your answers to the other questions that I posed above. Of course, a thorough consult including your breast history and exam with a reputable plastic surgeon is imperative. Regarding future pregnancies.....no problem and in fact if you could normally breast feed, the surgery should not impair this, and pediatricians encourage it. There are no plastic surgery contraindications to breast feeding after breast augmentation. Long term, over the course of aging, gravity, loss of elasticity, weight fluctuations, pregnancies, breast feeding, and hormonal changes, your own natural breast tissue will change, your implant typically will not, with the exception of the possibility of capsule contracture and rupture. Good Luck!