This could be pregnancy discrimination, depending on the circumstances of your particular case. In general, any pregnancy-related benefits cannot be limited to those who are married. This includes health insurance that reimburses expenses.
There have been a few recent decisions about pregnancy discrimination where the courts have allowed an employer to terminate an employee who has never been married because of her pregnancy. In these cases, the employer was a faith-based company, the employee signed a contract that contained a morals clause that required following this faith’s teachings about sex outside of marriage, and the pregnant employee had direct contact with children.