can an employer legally ask me to resign because i am pregnant, discrimination under title vii of civil rights act

More facts are needed, but pregnancy discrimination is generally a violation of legal prohibitions against sex discrimination. (Title VII of the Civil Rights Act of 1964) In the case you describe, you might be asked to resign from a particular position if pregnancy would render you unable to perform the job. (This is rare unless your doctor has given you medical-related work restrictions.) If your employer has a job you can perform within any restrictions, you should be transferred to that job rather than fired.

If this has happened to you, you may want to contact your local EEOC office immediately to try to resolve the situation.