Firing a pregnant employee–how should we proceed?

Q: “I own a temporary staffing agency. I have an employee at a client location who became pregnant in January 2014. She is now showing up late for work and also spending time not working or performing her job duties. We have documented and advised her of her tardiness. At this point, we are wanting to terminate our employment with her due to non-performance. Can you please advise?” – Joey, CA