The law requires that employers treat pregnancy the exact same way they treat other short-term disabilities. The law does not require an employer to have a pregnancy leave policy.
When determining if someone is being discriminated against due to pregnancy, the courts look at how the employer treats employees who have serious illnesses or injuries. So, if the employer does not offer unpaid time off to employees with illnesses or injuries, you probably do not have a pregnancy discrimination case.
You may be covered by the Family Medial Leave Act (FMLA), though, which can provide a limited amount of leave that is unpaid. Even under the FMLA, you would probably need to use all your vacation time and your sick days first before being allowed any unpaid leave.