How to make fiancé understand body changes

How do I get my partner to understand that my body will not just go back to normal after the baby. I’ve tried explaining it and he doesn’t get it . He thinks as soon as I have the baby my body will snap right back after 3 months and nothing will be different. I know my body will never be the same I’ll have saggy breasts and a squishy mom tummy and some extra pounds I’ll struggle to lose . I’ve accepted it , why cant I get him to understand it ?

Side note: His mom isn’t any help 😂 she is partly why he thinks it she said I’d be back to normal in 3/4 months and that she wore her pre pregnancy clothes home from the hospital. We are two separate people who are built completely different though and I kind of feel like maybe she just doesn’t remember it correctly because she also mentioned that I shouldn’t bleed for longer than 2 weeks and I know that can’t be true ( I have no idea , it just doesn’t sound likely)