Final answer:
The Pregnancy Discrimination Act of 1978 makes it illegal to discriminate against pregnant women in the workplace.
Step-by-step explanation:
The act that makes it illegal to discriminate against pregnant women is the Pregnancy Discrimination Act of 1978. This act is an amendment to the Civil Rights Act of 1964 and it prohibits job discrimination against women who are pregnant, planning to get pregnant, or returning to work after pregnancy.