Which hand should a promise ring be worn on?

My boyfriend bought me a promise ring for Christmas. We’re seven months pregnant with our first, are madly in love and plan to get married. We’re waiting to get engaged because we want to buy a house first but he wanted to do something special for me this Christmas so he bought me a beautiful ring and told me he promises to love me forever and only me 💑 only thing is is I have no idea what band to wear it on😂 we’re engaged to be engaged but it’s not official so I’m thinking right but then again I feel like a ring from someone you plan to be with forever is much more special than a class ring or a ring from a friend or a family member. Let me know what you guys think and why!!