If I want to lose 20 to 30lbs after my breast augmentation, how will it affect the size and shape of my implants?

Unfortunately there is not a set answer for this question. How much breast volume lost is dependent on the percentage of fat which makes up the breast.  Some women have a higher proportion of fat to breast tissue compared to other woman. These women have greater fluctuations of breast size when their body weight changes.  Generally it is different for each individual.

Thank you for the question. Although weight loss will not have a direct effect on your breast implants, significant weight loss may affect the appearance of your breasts in general.It is difficult to predict how a specific young lady's breasts will change with weight loss; how the breasts were affected historically with weight loss may be the best indication how you will do.Generally speaking, significant weight loss can lead to some change in breast tissue (and/or nipple/areola) position; this “sagging” is called breast ptosis medically.Sometimes, patients who lose weight will have a smaller amount of breast/soft tissue coverage overlying the breast implants; this change in breast anatomy may lead to a greater degree of breast implant palpability and/or rippling. In these cases, the breasts may also appear smaller.Therefore, generally speaking, it is in patient's best interest to achieve a long-term stable weight prior to undergoing elective breast surgery.I hope this, and the attached link, help.

Your breasts could look saggier or look larger than what you'd like on your frame however it is difficult to say for sure. How much do you weigh now? Generally speaking, the thinner you are at the moment, the more this weight loss will affect your results. That is why it's always better to be at a stable ideal weight prior to surgery.