Is it illegal for any employer to ask if a female applicant is pregnant?

Let’s say in America, or wherever that you kow about, is it illegal for any employer to ask if a female applicant is pregnant? Or does it depend on the kind of task te employee is expected to do? If yes why and if no why not?

I mean does it depend if the job is unsuitable for the safety of fetus and or not? This http://www.fluther.com/196882/usps-holiday-hire-and-asked-to-resign/ OP is what reminded me of the question.
The employer can just tell you the conditions of the job not ask you if you’re pregnant. Please ignore this question. I wanted to cancel it but it’s not possible.