Can 550cc High Profile Breast Implants Look Natural?

Only you and your plastic surgeon can tell what is best, but the general rule is that high profile implants aren't able to produce the natural looking slope in the upper part of the breast for most patients.

However, if you truly want to go up a full cup size then an increase of 200 cc might be in the range of what is needed. With your external diameter of 14.6, the diameter of the Allergan HP 550 is appropriate. But that is big for someone your size, and you would need to accept that.

Natural is both shape and how the size of the breasts balances with the rest of your body.

It’s always
difficult making decisions about breast size without performing a physical exam
or seeing preoperative pictures.In
addition, it’s important to understand the patient’s aesthetic goals.Each patient has specific anatomic findings
and unique aesthetic goals regarding size and shape of the breasts.All of these factors should be considered
when choosing the size of breast implants.Even when care is taken
preoperatively to determine a breast size, the final decision needs to be made
in the Operating Room by the surgeon.At
this point, an assessment can be made regarding how the patient’s breast tissue
interacts with the breast implant.In
some cases, when larger implants are utilized, patients can loose a natural
look and appear artificial.Occasionally, obtaining a specific size doesn’t allow the surgeon to
meet the patient’s other aesthetic goals.For these reasons, locking into a specific size isn’t always a good
idea.Results are often better when a
surgeon has the ability to make adjustments in the Operating Room.In your case, it’s important to
thoroughly discuss your aesthetic goals with your surgeon.Make sure he has the latitude to do what’s
best for you in the Operating Room, so he can meet your aesthetic goals.It’s important that you have an approximate
idea of the breast implant size that you’re considering, but don’t become
married to an absolute number.It’s
important to remember that going with an implant that’s too large can result in
an artificial appearance.

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.