I'd rather someone ask if we were trying than if it was an accident (which is basically the same question, but the wording is nicer). I had a stranger ask me if I was aware that I could have had an abortion, since I was soo young and not married... I'm 29, lady, keep your mouth shut, maybe?
Some people have absolutely no filter, and I never even thought to ask someone if their pregnancy was planned. I might wonder, and even think "Well, that went fast" but in the end, it's none of my business. I won't embarass myself or someone else for the sake of "small talk"...