While FMLA does not apply because of the size of the employer, and would not apply because of the length of employment, California has a state law which is applicable to any employer who has 5 or more employees, which requires up to four months of leave for pregnancy/disability. It doesn't matter how long the employee has worked there; she is automatically entitled to this leave. Terming her employment due to taking this leave would be a violation of the law.
Terming ANY employee BECAUSE she is pregnant would be a violation of Federal law.