Is it considered unlawful to undergo cosmetic procedures to improve one's physical appearance, such as a tummy tuck after childbirth or breast augmentation to restore a small breast to its natural size, if the woman wears a hijab and undertakes these procedures solely to beautify herself for her husband?

The cosmetic surgeries mentioned in the question are not permissible, even if they are for the husband only, because they may result in altering Allah’s creation. This is unless the disfigurement is clear and deviates from the norm, or if the improvement is achieved using medical drugs or exercises that do not cause harm.