Suggestions for Silicone Breast Implant Size, 5'6" & 130 lbs. with saline 375 mentors

To go from a D to a C is achievable.  Dropping 75-100 cc is an old rule of thumb to go down a cup size, so 339-371 would be in the ballpark.  Option 1. Tell your surgeon what you want to achieve and trust him/her to use intraoperative Sizers and judgement to achieve your preoperative instructions.  Option 2.  Have a preoperative deflation performed, then try on the various implants and agree on a plan with your plastic surgeon. I like to empower patients with this choice if they can live without implants for a day or two. When we agree on a plan ahead of time, I am able to get patients almost exactly what they desire.  

It is very difficult to determine the exact size and shape of implant that will get you the results you desire without an examination by a board certified plastic surgeon. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be tough to answer your question. For example your native base width of your breast will determine in many cases the maximal volume per implant profile that you can accommodate. To illustrate a 100 cc difference may make a significant difference with a narrow base width, but much less of a difference if you have a wide chest wall and wide breast “foot print”. Additional critical decisions will also be made by your plastic surgeon such as: whether your implants will be above or below your pectoralis muscle. These choices are recommended to you based on the look you desire, the amount of sagging you may have, and other deciding factors. I always find it helpful for patients to bring in photos from my or other plastic surgeons’ websites to illustrate what they would like to look like. I bring these photos to the operating room for reference during surgery so that I have the advantage of “seeing through my patient’s eyes” to achieve their wishes.I personally order more than one set of implant sizes and use sterile sizers placed in the pocket during surgery to know in advance exactly which implant would work best for you. Software morphing programs can be helpful in some but not all cases (e.g. doesn't work well in my experience with existing implants, sagging or asymmetric breasts).My advice is to accept advice from your board certified plastic surgeon after a formal consultation and examination with measurements.. If still confused don't hesitate to get a second opinion.

It is very important to communicate your size goals with your surgeon.  In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or “C cup” or "fake looking" means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on who makes the bra; therefore, discussing desired cup  size may also be inaccurate.
I use  intraoperative sizers and place the patient in the upright position to evaluate breast size. Use of these sizers also allow me to select the press implant profile (low, moderate, moderate plus, high-profile) that would most likely achieve the patient's goals. The patient's goal pictures are hanging on the wall, and allow for direct comparison.
I have found that this system is very helpful in improving the chances of achieving the patient's goals as consistently as possible.
By the way, the most common regret after this operation, is “I wish I was bigger”.
I hope this helps.