Petite 5'1", 110 lbs, 32C. Is it possible to get natural looking DD?

Thank you for your question and photos. Based on these photos, you can go to a DD cup. A cup size in your case is around 150 cc, so an implant in the 300 cc range will get you there. Things to consider about outcome is your chest wall anatomy. Superiorly is quite prominent over the sternum, and it dips just about the superior poles of your breasts into a depression. What this means, that you will not have a close cleavage. In my practice I use the Vectra 3D to simulate the augmentation to give you a realistic visual expectation. BTW, round or shaped implants, no one can tell the difference if the surgery is done correctly. Both implants "act" like shaped implants in the body.
See a board certified plastic surgeon for an in person consultation/evaluation. Good luck.

I recommend an in person evaluation by a plastic surgeon to discuss your goals and be measured. After I measure and discuss goals with my patients, we then look at pictures of women (my patients) with similar measurements with different size and style breast implants. They also get to see what they would look like when the breasts have fully dropped and fluffed and in clothes. You get to see actual results and the surgeon's experience and not a virtual idea of what can be achieved. Women tell me that this process is very helpful in determining what size and style breast implant is right for them. You should also be aware that results of surgery can change in the future with significant weight change and/or stretching of skin from the milk of pregnancy. Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author. RealSelf Distinguished Hall of Fame

Breast implant size has nothing to do with bra cup size - which varies with each bra manufacturer. The average implant volume placed in the US is 350 cc. A very general estimate is that one cup size is 150 to 200 cc of volume, but realize that no direct correlation exists between the two.
Your board certified plastic surgeon will be able to guide you to the correct size implant. Based upon measurements of your chest wall and breast imprint width as well as knowledge of all breast implant sizes and profiles should make this process easy. Try on a profile bra with various silicone cc sizers your PS recommends under a tailored blouse, tank top, bikini, workout clothes, and other garments you typically wear so that you have the best idea of whether the garments fit as desired with the implant selected. Select the size that boosts your self-esteem, and fits your anatomy. Choosing an implant based on an aesthetic goal that exceeds your anatomic capacity may lead to complications and possibly revision.
In general, the risk for infection and capsular contracture may be greater for armpit and areolar incisions for different theoretical reasons. I prefer the transumbilical or periareolar for saline and the IMF or periareolar for silicone. The pain is due to placing the implant under the pectoralis major muscle, and I do not think any of these incisions is necessarily less painful.