Breast enlargement through nipple or under the muscle, which do doctors prefer and why?

I recommend an incision through the fold to minimize your complication rate.   I recommend an in-office examination as well as a detailed discussion with a surgeon who you are comfortable with.   Finally, make sure your surgeon is a Double-Board Certified Plastic Surgeon certified by the American Board of Plastic Surgery (ABPS), a member of the American Society of Plastic Surgery (ASPS) and the American Society of Aesthetic Plastic Surgery (ASAPS).Best,Dr. DesaiBeverly Hills Institute for Aesthetic Plastic SurgeryHarvard Educated, Beverly Hills & Miami Beach TrainedDouble Board-Certified Beverly Hills Plastic Surgeon

Hello,The data is conclusive that inframammary scars have the lowest capsular contracture rate, the number one complication of breast surgery.  Periareolar scars do not deserve their reputation as being inconspicuous. After nearly two decades of specializing in revision surgery, I can assure that the average periareolar incision is more conspicuous than the average inframammary incision. Subpectoral placement leads to lower complications, especially long term, than over the muscle. Best of luck!

You have posed a number of very commonly asked questions...When undergoing breast augmentation there are a number of choices which need to be made: saline or silicone? Volume: Larger or smaller? Incision? However, one of the most commonly debated choices is that of implants placement: subglandular/ submammary vs. subpectoral/ submuscular? While many surgeons recommend submuscular placement there are distinct differences to each approach.
 Subglandular Augmentation (“overs”): 
Subglandular augmentation means place of the implant underneath the breast tissue but above the pectoralis muscle. Subglandular placement spares the pectoralis muscle which leads to reduced post operative pain/discomfort and no impact on muscle function post augmentation. Recovery is also faster.Subglandular augmentation can impact mammographic evaluation of the breast. However, as dedicated breast radiography has become more prevalent this has become less of an issue. Fellowship trained radiologists have become familiar with evaluating breasts post augmentation. It is also important to note that implant position does not interfere with visualization of breast tissue via contrast enhanced MRI (the most sensitive and specific study available for breast cancer detection).Studies suggest there is an increased risk of capsular contracture when implants are placed in a subglandular space.Aesthetically, implants placed superficial to the pectoralis major create a rounded, convex appearing breast profile. This effect is camouflaged, at least initially in larger breasted patients. However, as a woman ages fat atrophies and breast tissue descends. The result is a more noticeable implant specifically in the upper pole. Similarly, patients who have thin coverage superiorly are more likely to be able to perceive the implants and at higher risk of visible rippling when compared to subpectoral augmentation.  Subpectoral Augmentation (“unders”): 
Subpectoral augmentation is technically a bit of a misnomer. Traditionally, subpectoral augmentation involves the release of the pecotralis major muscle from its lower attachments. This allows the muscle to “window-shade.” The upper hemisphere of the implant sits underneath the muscle (dual plane). This release contributes much of the discomfort encountered postoperatively by patients.Subpectoral implants have a lower rate of capsular contracture.Aesthetically, in contrast to submammary implants (which are prominent in the upper pole- especially in thinner patients), the pectoralis muscle both conceals the underlying implant and flattens the upper pole. This flattening effect creates a natural sloping as one proceeds from the upper portion of the implant to the lower portion.The most commonly cited drawback to sub-muscular augmentation is the animation deformity associated with contraction of the overlying muscle.In my practice, all things considered, sub-muscular is the plane of choice.The choice of incision in the setting of breast augmentation is one which is made jointly by both the patient and the surgeon. Surgeons typically have their preference as do patients. 
Your options include-
The direct approaches:
-IMF (fold)- (+) popular, direct access with good visualization, if placed appropriately it is well tolerated and well camouflaged; (-) the scar can migrate as the fold descends with augmentation, the incision can be misplaced
-Infrareolar- (+) commonly used alternative to the IMF incision, the location is more consistent and predictable than the IMF, especially useful in those patients with a poorly defined fold; (-) there is a limitation based on the size of the areola (smaller areolas will limit incision size and thus impact visualization), this technique is also associated with slightly higher rates of contracture
The indirect approaches:
-Transaxillary (armpit)- (+) avoids a scar on the breast mound but must be performed endoscopically as there is no direct visualization of the pocket during dissection otherwise; (-) can be limiting with regards to implant size and type (although the Keller funnel has helped)  
-Transumbilical (belly button)- (+) avoids a scar on the breast mound; (-) no direct visulization of the pocket, saline implants only.
Ultimately, no approach is perfect. They are different as is each surgeon. The incision decision is best made in concert with your board certified plastic surgeon (ABPS).