Do breast implants placed under the muscle start sagging after a while?

Thanks for your question. Assuming that you have not recently been pregnant, breastfed or changed your weight the your breasts are unlikely to quickly become saggy after breast augmentation under the muscles. All implants will increase the weight of your breasts, and stretch the skin somewhat, so in the long run they can sag slightly. However by putting implants under the muscles, the weight of the implant is largely held between the muscles and the chest wall. If an implant is above the muscles, when all the weight is on the breast skin and sagging and bottoming out is more likely.

Thank you for your question.
After a breast augmentation, the implants settle into position over a period of a few months. Beyond this, the breasts may sag with age, time, pregnancy & breast feeding and fluctuations in weight. Implants under the muscle retain their position better while the breast tissue which overlies the muscle may droop with time. If you are able to attach a picture, it may help appreciate your particular scenario better.

The two main reasons for breasts to change after cosmetic surgery are significant weight change and/or pregnancy. It is the milk of pregnancy that can possibly stretch the skin and change its volume, not the implants.Dr. Ted Eisenberg, Board Certified Plastic Surgeon and Author

Most experience slight sagging after a while especially with large implants. However, under the muscle implants are less likely to sag that much as the weight of the implants are held by the muscles and underlying tissues.