Breast implants turn into flat squares when I flex pecs. 3 weeks post-op.

Thanks for your questions. Submuscular breast implants can look a bit strange early after surgery. Odds are that they will improve greatly. The time it takes depends on many things. This includes your original anatomy, the implants which were used, the size if the implants, and your surgeon's operative technique. Give it some time and see what happens. Keep talking to your surgeon and see what if anything needs attention. Don't expect any revision until three to six months.

Breast augmentation is most commonly placed under the muscle for multiple reasons, however occasionally patients do benefit from the implant placed over the muscle.

Let’s start with reasons why the breast implants are placed under the muscle. The first, is that the risk of capsular contracture has been found to decreased when the implant is placed below the muscle. It is believed that the constant motion of the muscle keeps the implant moving just subtly and thus keeping it soft. The second reason is that the muscle provides additional coverage over the breast implant, giving it a softer feel and decreases the appearance of rippling from the breast implant.

The new gummy bear (highly cohesive) breast implants have helped with many of the issues that placing the implants under the muscle have treated. The gummy bear implants have decreased the rippling that can be seen when compared to saline breast implants. Also, the advent of the Keller funnel and antibiotic solutions used during a breast augmentation have decreased the risk of capsular contracture.

The benefit of placing the breast implant in the subglandular (above the muscle) is that it prevents something called animation deformity. This is when the breast implant moves as the muscle is flexed. Remember a little movement is great to prevent capsular contracture, however if the implant moves a significant amount it can be disturbing to the patient. If the breast implant is over the muscle, then there is very little movement of the implant when the muscle is flexed.

I typically will place the gummy bear breast implant over the muscle if someone has animation deformity. This will reduce the movement of the implant when the patient is moving and flexing. Bodybuilders and other patients that are at risk of animation deformity I may also place the implant over the muscle during their breast augmentation to avoid this issue.

There are pros and cons to both a subglandular and submuscular placement of the breast implant. It is important to discuss both of these options during your consultation with your Board-Certified Plastic Surgeon. The highly cohesive breast implants have given patients and plastic surgeons more flexibility in the placement of the breast implant.

Dear cookiesanddough,I understand your concern and it is quite normal to be concerned about the outcome following a breast augmentation surgery. However, you are still in the very early stages of the healing process at 3 weeks  post op.Your will need to be patient at this moment and follow what your surgeon has instructed you to do post operatively.At the early stage of the healing process, the tissues around the implants are still swollen. Once the swelling subsides and the implants drop to their final position, you will be able to see the final result of your breast augmentation surgery.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

It is normal for your breasts to feel toght three weeks out from surgery. The muscles will soften and the implants will settle as more time passes 

All of what you surmised may be playing a role. While there is no guarantee it will totally resolve, it may and it is highly likely to at the very least improve over the upcoming months.

You have animation deformity which is commonly seen with sub-muscular implantsIt should improve with time but will not go away completely

They go flat and sqarish because your muscle crushes them against your ribcage. This happens with submuscular implants.  This annimation does not go away when implants are behind muscle. 

Implants placed under the muscles will always have some degree of animation deformity when you flex your chest muscles. In some cases the deformity is severe enough for patients to consider revision surgery. The only way to eliminate the animation deformity entirely is to place the implants above the muscles. There can be some reduction of the animation issues with a dual plane split muscle pocket, as well. If you still have concerns after 3 months, then discuss this with your plastic surgeon. Best wishes.