The trial court's order granting the conservator's petition for an order authorizing her to consent to a hysterectomy and oophorectomy on the conservatee's behalf is affirmed, where: 1) Probate Code section 2357 governs because the objective of the proposed surgery is to treat the conservatee's medical conditions, not to prevent her from bearing children; 2) the trial court erred in applying the preponderance of the evidence standard of proof because the proposed surgery would have a substantial and irreversible impact on the conservatee's fundamental right to procreative choice; but 3) the conservatee's counsel failed to show it is reasonably probable the trial court would have reached a different conclusion if it had applied the proper standard, and therefore failed to show the error was prejudicial.