Why do i feel the need to get pregnant?

I have already asked this question, but I'm not fully satisfied with the answer. I have been with my boyfriend for 3 years & there has been two seperate times where I have had this feeling. The feeling eats at me & drives me crazy. I feel like a just have to get pregnant. I definately want to be a mom, but he is not ready. I'm on mirena so we should not accidently get pregnant. Anyone else had this weird feeling? What did you do to stop it? I just don't think he will budge on this one. My heart feels like its breaking because I want this so bad. HELP!