Im going to just come out and say it, your partner probably already knows you stutter, and it hasn't come into conversation because they are just being polite. Talking about it will break some ice, they may have questions, they may think you're nervous. Talking about it is the right thing to do. A friend of mine after knowing me for a few years, finally had the opportunity to ask me "Would you rather I help you finish your sentences, or would you rather I let you finish yourself" It was the first time it came up in conversation, I answered him, he asked a few more questions, then it was always easier to talk to him after that.