The Emily Post Institute says Ms. is the default address for women in America. Use it if you don't know a woman's marital status, or she is married but hasn't made her preference clear — it should offend no one. But Ms. has only been used for the last 40 years.
The abbreviation first appeared as a space saver on tombstones and in headlines, and it was not meant to indicate marital status or, the euphemism preferred at the time, "domestic situation." But in 1901, The Republican, a Massachusetts newspaper, advocated Ms. as a new term.
Clearly, what is needed is a more comprehensive term that does homage to the sex without expressing any views as to their domestic situation, and what could be simpler or more logical than the retention of what the two doubtful terms have in common. The abbreviation "Ms" is simple, it is easy to write, and the person concerned can translate it properly according to circumstances.
Yet it didn't take off for another 70 years. Feminist advocate Sheila Michaels saw the abbreviation in 1961 and assumed it was a typo. In 1971 she suggested its use on a radio talk show to fill the void in honorific titles for women. Gloria Steinem was listening and decided it would be perfect for her new publication Ms. Magazine, catapulting it into the mainstream.
Even though the US government approved Ms. for official documents in '72, opponents of nonsexist language resisted its use until Congresswoman Geraldine Ferraro settled it once and for all when she was the 1984 vice-presidential candidate. She went by her maiden name, though married, and pointed out that Mrs. would not be correct — as Ferraro wasn't her married name — and Miss was incorrect, too, as she was married.
Now I wonder if she started the trend of keeping your maiden name after marriage, too?