If my BWD is 11.5-12cm can I still pick an implant with a 12.5cm width?

Hello. The dimensions of the breast are important in implant selection as you know.The other breast dimensions such as skin stretch, nipple to fold distance and general skin laxity also play into implant choice.  A base diameter implant of 12.5 cm would probably fill your breast but perhaps a higher profile implant could be the answer.  In the Allergan line, there are many different profiles available in Natrelle Inspira (SRM--mod, SRF--full and SRX- very full projection).  In the Mentor line, which I think the Moderate Plus style is, I think there is a higher profile implant is available HP. The higher the profile, the more volume fill with the same base diameter. The more projected the implant the narrower and more pointy the breast will be.
Hope this helps.
Dr. Hamori

the answer is yes but the implant will be on the wider end of the range for your breast, causing some "outside" push on the implant.  Be sure to discuss your goals and concerns with your board certified plastic surgeon and consider the pros and the cons. Be certain your surgeon is properly certified.  

Nothing is more reassuring, than hearing from a RealSelf user who is thinking in terms of diameter rather than "cc." 
Patients often think in terms of cup size when considering augmentation. Unfortunately, devices are sized in terms of milliliters (cc) of volume. This can lead to some confusion when sizing. Additionally, it is important to remember that cup size itself is not standardized with variations from one manufacturer to another. Unfortunately, as many women can attest their cup size in an industry leader such as VS is not necessarily transferable to another brand.
Another point which is often under appreciated is that of anatomy and starting point. Any implant will add volume to the volume which is already present. The implant is additive. A particular volume will not necessarily confer the same cup size to different patients (often times it will not even confer the same cup size to different breasts in the same individual...remember they are "sisters" not "twins"). 
A general rule of thumb is that 125cc can represent somewhere between 1/2 to a full cup size increase. Smaller volume differentials (25-50cc) are typically less consequential representing a volume change of less than a shot glass. However, I have found these numbers, at least anecdotally, to be of little help. Patients often present with notions/goals which do not correlate with these sorts of sterile volumetric assessments.
When sizing patients, there are a number of useful tools including:
-3D imaging (has the added benefit of offering a volumetric analysis of the pre-operative breast)
-Breast sizers (rice bags)
-Goal photos
I also recommend that patients commit to a particular look rather than a cup size. Once a patient settles on a look that pleases them the overall cup size increase becomes less relevant.
With regards to your particular question, in the setting of a round gel staying within 1 cm +/- of your measured width is advisable (with a preference towards "-" so as to avoid the dreaded lateral show and "fake" look). A 35 cc difference is not significant (less than a shot glass). 12.5 cm is 1 cm greater than your measured width and should be acceptable. However, patients should remember exceeding your BWD places one at greater risk of lateral fullness. If that is a major concern, err on the conservative side with regards to volume. Alternatively, choose a higher profile device width a lower base width and greater projection.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

Hello and thank you for your question.  The
best advice you can receive is from an in-person consultation.  We use base diameter as a guide for implant selection, but it is not a strict rule.  You may also want to consider a high profile implant, which has a narrower base width.  The size, profile, and shape of
the implant is based on your desired breast size/shape, your chest wall
measurements, and soft tissue quality.  This decision should
be based on a detailed discussion with equal input from both you
and your surgeon.  
Best wishes and good luck.
Richard G. Reish, M.D.
Harvard-trained plastic surgeon

Thank you for your question. "Tissue based planning" relies on your chest and breast measurements and is the safest way to ensure good long term results. Your surgeon should determine what size and projection are necessary to give you your desired result based on your body's measurements. I think it's best to choose an implant more narrow than your base width. The best way to assess and give true advice would be an in-person exam. Please see a board-certified plastic surgeon that specializes in aesthetic breast surgery. Hope this helps and good luck with your surgery.