I am a 34B and thinking of getting 375CC implants under the muscle, what size would that make me after they've settled? (Photos)

Hi.  It is hard to say but I would be willing to guess that would put you into a full D cup size range as you look like you are likely a fairly full B cup currently.  In general that is a fairly common size range that you are considering and certainly seems reasonable based on your picture.  I would recommend consulting with a board certified plastic surgeon with good experience with breast augmentation.  It would be helpful likely for you to try on sizers to get a general idea as well.  Good luck and take care, Dr. Howell

Breast augmentation, sometimes referred to as a "breast aug" or "boob job" by patients, involves using breast implants or fat transfer to increase the size of your breasts. This procedure can also restore breast volume lost after weight reduction or pregnancy, achieve a more rounded breast shape or improve natural breast size asymmetry.

Subglandular placement is accomplished by positioning the implant under the breast glands, but on top of the chest muscles. Very few breast augmentation patients have enough breast tissue to permit a subglandular augmentation.  
It is not unreasonable to increase the size by about 10% due to compressive forces if breasts implants are not placed over the muscle.  Submuscular is a better choice for a number of reasons including reduced risk of capsular contracture, better longevity, and greater implant coverage. 
There is no correlation between cup sizes and number of cc's.  Bra manufacturers do not follow the same sizing specifications.

At size 34, each 100 cc of implant corresponds to 1 cup size change.  375 cc implants would take you up almost 4 cup sizes to size F.  Implants this large are mechanically unstable and will most likely lead to complications.  Implants this large will not fit retro-pectorally and will descend inferiorly and laterally requiring revision.  I strongly recommend you consider small implants or fat transfers.Best Wishes,Gary Horndeski, M.D.

Hello, thank you for the question.
The size you gonna have after surgery depends, exactly exactly this is hard to know. But I guess you could get a C full or D size. Hope this helps.
Dr. Luis Mejia.