I never say it to people I don't know - precisely BECAUSE I don't know the situation. Maybe she did miscarry. Maybe she's infertile and not happy about it. Maybe her mother just died and she doesn't want to think about it. A lot of people do say happy mother's day to everybody or all women, but I've always thought it was safer to only say it to people one knows at least enough to know whether or not it's appropriate.