Will a weight loss of 20 pounds lead to saggy breasts?

Although I understand your concerns, I would suggest that you make   achieving your health goals a priority;  hopefully, at your young age a slow weight loss will not lead to too many changes in your breast size/position.   Obviously, good overall health will be of much better value to you long-term than maintaining a specific breast size/position. Best wishes as you work towards your goals.

Christie.   In some people, significant weight loss can cause breasts to sag, but doubtful at your age with a 20 lb. weight loss. The real question is do you really need to lose that much weight? Height plays a factor. Talk to your family doctor about your desire to lose 20 lbs. Good luck!