Wanting a baby

Why so I have such a strong urge to have a baby it almost hurts emotional and physical. I know im ready to have a baby I know in my heart I am, but I don't understand why I have the uncontrollable want?. My fiance doesn't want one yet and I'm not sure when he will want one. We're still young but I've always wanted to have a family young * I'm 19 he's 21* please help me figure this out I literally break down and cry because I just want to have a baby so bad but he doesn't.