Can natural hormone replacement therapy cause weight gain?

Yes: Unfortunately some patients do gain weight (often in their abdomen, breasts, and back of their upper arms) with estrogens, whether or not the preparation is "bio-identical" or a synthetic analog. Balancing Estrogens with Progesterone and/or androgens often reduces this effect, which is dose-dependent. Prior weight gain to birth control pills sometimes predicts the same weight increase with hrt.

Yes if unbalanced: The key to natural hormone replacement is balance. If balanced you should gain weight no more easily than you did when you were younger if you are eating a proper diet and exercising to maintain adequate muscle mass. Muscle mass controls your metabolic rate. Too much Estradiol or Progesterone can cause weight gain. Too much testosterone can boost appetite.