Sleeping n pregnant

Ok ladies. My fiancé won't let me sleep on my belly. Like he so over protective over his son n it's annoying. When we found out I was pregnant at 5 weeks... He jus been annoying to me. Good on ways of getting me food n ect but I have always slept on my belly. I'm 18 weeks now n I still turn on my stomach when he goes to bed. I hardly look pregnant at this moment. I went from abs to the abs are gone to a bloated look right now.  He thinks I'm harming the baby. Am I?? How long will I be able to stay on my belly?