I was told by PrincessPeahead (who is The Quality) that black is never right for weddings. However I ignored her completely and wore black to a wedding when I was six months pregnant and no-one batted an eyelid!
depends on the culture. i remember years ago, getting ready to go to a mmuslim weddin in birmingham, and shocking my french flatmate by ironing a black outfit.
she was soooo shocked, it was quite comical.
You can hire fab maternity dresses at a shop in Covent Garden (not sure where you are but they may well do it in other places too) called, I think, bumpsadaisy or something vaguely irritating like that.