All boobs sag real or fake and the more weight you add into your chest the faster it will happen. Its not a case of if its a case of when but you can help it by wearing a properley fitted bra, using a sports bra to sleep in, always wear 2 bras (one being a sports bra) in the gym and never go braless (unless for short periods) and try to keep your weight stable. Breast implants have a limited life anyway so when you get a BA you always need to remember that you will need further surgery in the future and if that includes an uplift then you can worry about that then.