Is there any way to know what size sports bra I should buy if I'm looking to go between a DD and DDD cup?

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.The key to obtaining a natural result is to stay within the parameters defined by your BWD. 
With regards to your specific question, predicting cup size is impossible. Implants of similar volumes will confer different cup sizes on different patients. This combined with manufacturer variability complicates this sort of prognostication.
The best advice is to "love the look...not the cup."
As always, discuss your concerns with a board certified plastic surgeon (ABPS).Donovan Rosas MD
Board Certified by the American Board of Plastic Surgery
Member: American Society for Aesthetic Plastic
Surgery, American Society of Plastic Surgeons
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Hello. Thank you for your inquiry. This will be dependent on your current band size as well as the implant size plus your current breast tissue.
It is important to have an in-person assessment regarding your concerns as it is difficult to determine virtually without any photos.

There are many different factors that go into choosing a breast implant size which will correspond with your body dimensions while concurrently achieving your aesthetic results. Some of these factors are: Implant size (CC's), Implant profile (Low, Moderate, High), Placement of the implant (Subglandular vs. Dual Plane) etc. We utilize smooth round cohesive silicone gel 'gummy bear' implants for the best results. 

During a consultation your aesthetic goals are heard and taken into account with the measurements of your breasts and the integrity of your tissue, as well as level of ptosis (sag). From here we collectively can find your 'perfect fit'. 

You are able to try out sizers and different implants in our office such that you can get an idea of what size you would be post-operation. We also offer Crisalix 3D plastic surgery virtual simulator during our in-person and virtual consultations to preview potential results. After your consultation, you will receive your very own implant sizers to take home with you (at a small cost) whether or not you decide to go forward with your procedure.

 I usually recommend that my patients stop smoking, take vitamin C daily, and avoid aspirin/ibuprofen and vitamin E 3 weeks prior to surgery. 

I specialize in the 24-hour Recovery method which minimizes trauma to the breast and decreases pain post-operatively.

We encourage you to look at our website and Instagram (drkaraplasticsurgery) for lots of before and after pictures!

Please note that this response does not substitute patient-doctor relationship.

We welcome any and all patients seeking advice to come in for a complimentary consultation available for limited time only.

During the COVID-19, we are offering complimentary online consultations as well as in-office consultation with social distancing and masks. Procedure done in-hospital private surgical facility while taking covid-19 precautions. We utilize UV-light to clean the air of viruses and bacteria during our consultations and surgery.

We invite you to come in to discuss your options more thoroughly.  Please send us an email at info@drkaraplasticsurgery.com with the subject line: "Attention Meagan" and we will be happy to coordinate your appointment!

M.Kara,MD,FRCSC (27+ years experience)

- Specializing in 24 hour recovery/ out to dinner breast augmentation in-hospital private surgical facility for your added safety and peace of mind