Im a Small B Cup Will 340cc Make Me a Large D Cup or Should I Go Upto 380?

Thank you for the question.

Unfortunately, there is no direct correlation between  the size of breast implants  and the resulting cup size. In other words, no surgeon can promise that a certain breast implant size will result in a certain bra cup size.  I have found that trying to make this correlation is one of the most common sources of patient dissatisfaction after their breast augmentation procedure.

In planning your breast augmentation procedure it will be very important to communicate your size goals with your surgeon.  in my practice I find the use of goal pictures to be very helpful.  in your case you could use your own before breast-feeding pictures as the goal. I have found that the use of words such as “natural” or “large D cup” or "fake looking" or "top heavy" 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 breast 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.

Without an exam it is very difficult to suggest what may be the possible cup size after implantation.  Vup size is very variable.  In general, a "B" cup will probably go up a cup or maybe two with that size implant.

Cup size, as has been said here by others, is an entirely unscientific and bizarre system which has essentially no consistency among bra manufacturers, or even from country to country. Ultimately, you are better served by helping your surgeon understand the degree of projection you want, either via pictures or using sizers to estimate your target implant size. I never guarantee an exact cup size, but knowing that a patient wants to be more on the "C" side or the "D" side can help with decision making. You need a face-to-face, in person consultation with a board-certified plastic surgeon to adequately answer this. Luckily, there are lots of us!