baby conversation

I want a baby just one more and I'm so not sure how to explain how I feel to my fiancé I am a very self conscious person. I know he is unsure if he wants another one. We are great parents together we have 3 all together his two and my one I feel one more would be absolutely perfect and I would want to tie my tubes and be done. I love my man and don't want him to feel pressured and I don't want to be selfish because I have this strong maturnal instinct to have one more. I realize he may not understand this, how can I explain myself without feeling totally selfish?