My fiancé [F26] is overweight. How do I [M26] tell her that I need her to do something about it?

My fiancé is becoming increasingly overweight - and frankly she knows it. She doesn't do anything about it however. She used to be very adamant about eating correctly and working out, but she has stopped lately. It feels like any excuse she can use to not take care of her self she will use. I am worried about her health, want to find her more attractive, and frankly it's a turn off when she doesn't take care of herself. We are getting married in 3 months and I surely expected her to try to lose weight for our big day but that hasn't happened either. In the past when we have had similar conversations she just gets upset and shuts down. How can I talk to her about how important this is to me without being rude? I just want to help her. She used to care a lot about her looks/weight/health. I'm worried if she doesn't do something about it it will get increasingly worse, as it already has, and she will have health problems and complications. We plan on having kids in the next 3 years or so as well.

Edit: I am not a hypocrite, I am 220lbs, 6'7. I focus on my health and care about my wellbeing. We shared those same values when we starting dating.