My mum actually asked me about "what if you want to have another kid?" and what made her understand just how big a "no" those parts are to me was my response of "Seriously, if I get pregnant again, I'll get rid of it, and failing that, I'll go hang myself."
I said it in a half joking manner at the time, but it made her go "??" at me, which led to the discussion allowing me to explain just how much the idea bothers me.
Mind you, I don't think she understands fully yet because I tend to explain myself in a "matter of fact" way and don't display the emotion behind it.