As a pregnant woman in the workplace, you have certain rights, and two federal statutes protect these rights. Title VII of the Civil Rights Act of 1964 covers a broad range of types of discrimination that may take place in a work environment. It also covers the Pregnancy Discrimination Act of 1978, which dictates that any type of discrimination against a pregnant woman because of her condition is also a form of sex discrimination.
The other statute is the Family and Medical Leave Act of 1993, which grants parents who meet certain criteria up to 12 weeks of unpaid leave from the workplace without them having to fear losing their positions or insurance benefits. For the FMLA to apply to you, you must be employed by a company that also employs at least 49 others, and you must have worked there for at least one full year before taking your unpaid leave.
First, if you can still perform your regular job duties, it is up to you to share the news about your pregnancy at your own discretion, although you can, for obvious reasons, probably only conceal it for so long. Your employer cannot remove you from the workplace because of your condition as long as you are still able to perform. You also have protections in your right to return to the workplace after taking time off relating to your pregnancy and before your child is born, as long as you are still able to get your job done as you typically would.
In addition, your employer cannot force you to change your basic job duties if you are still able to fulfill them sufficiently. He or she also may not, for example, move you from a front-desk position to a more hidden environment so that clients or customers do not recognize your pregnancy status.
This is just an overview of some of the workplace protections you have as a pregnant employee in the U.S. If you feel your rights have been violated, consider consulting with a lawyer.