If a woman gains a few lbs her clothes will feel uncomfortable and she'll acutely know it whether you mention it or not, so don't think that she's ignorant of the fact that she's gained weight. If you mention it, the only new information you're giving her is that you're upset by it. So don't bother trying to be manipulative about it- either hold your peace and work on your own insecurities, or be honest and tell her "I don't like that you've gained weight. How can I help you lose it?"
If you really are concerned about her health, I suggest you forget about her physical appearance and focus directly on enabling her to have better food and exercise and sleep- cook healthy meals, buy fresh fruit and veggies, plan active dates, help out with her errands/responsibilities so she can be more rested and less stressed.
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