How does weight gain or weight loss affect the size of the breasts after a tram flap breast reconstruction?

Variable. Weight gain or loss would result in slightly larger or smaller tram flaps even after reconstruction.
It can change things. The reconstructed breast may get smaller with weight loss and larger with weight gain. Since the tram flap uses lower tummy tissue, the reconstructed breast can act like your tummy fat used to before it was moved to your breast. The degree of tram flap change with weight change varies from person to person. If your tummy never changed much with weight change, your new breast won't either.
The flap has fat.... The flap has fat in it so it can get larger with weight gain and smaller with weight loss. This does vary though. :).
It does. Significant weight gain or loss of 10-20 lbs or more affects the fat globally in the body. Since tram flaps contain mostly fat, the volume and/or shape can change with any weight gain or loss. It is much more important to stay healthy, maintain a balanced diet, and not to remain sedentary. This will not only maintain your results but also keep you living longer and with a better quality of life.

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See nutritionist. You say you're breastfeeding. It is normal to loose some weight while doing that. I don't know your numbers but go see a nutritionist and she can guide you on your dietary intake. Read more...