Employees don’t go from good to terrible instantly. There is usually a slow and steady decline. Be sure that the process is carefully documented, right from the very first verbal warning.
Recent case: Mindy Slater was fired from her job performing eye examinations. She believed the reason was that she had become pregnant and had complained shortly before her discharge about.
But her employer showed the court that it had been unsatisfied with her performance before she became pregnant. Her supervisor noted that he had first given her a verbal warning and then followed up with an email outlining her—all before anyone knew she was pregnant.
That was enough to get the case dismissed. (Slater v. Energy Services Group International, No. 10-14939, 11th Cir., 2011)
Final note: Wonder how to greet news an employee is pregnant? “Congratulations!” will suffice.