Soon after she passed the first trimester of her pregnancy, Michelle McCusker told her employers that she was expecting. The Catholic school teacher also mentioned that she did not plan to marry the child's father.

The principal "made it seem like it was fine," McCusker, 26, said. But two days later, she was fired. Officials said she had signed a contract to uphold core teachings of the Catholic faith, and that her behavior had violated that agreement.

At first, McCusker cried. Then she got angry. Last week, the New York Civil Liberties Union filed a complaint on her behalf, charging that the Brooklyn Archdiocese -- which runs St. Rose of Lima School in Rockaway Beach -- had violated a federal law banning discrimination against pregnant women.

"The school singled her out because she was pregnant, and the only way they could do that was because she was a woman," said Cassandra Stubbs, an ACLU attorney. "How do they determine if male employees engage in premarital sex?"

The conflict has sparked a debate over the constitutional rights of a pregnant woman vs. the rights of a religious institution to enforce its moral standards in the workplace.

McCusker, who appeared at a Manhattan news conference with her parents, has refused to divulge the identity of the father, saying only that he did not want to be involved in the birth and raising of the child. But she described the genesis of the case, and her feelings about her treatment by the archdiocese.

"I don't understand how a religion that prides itself on being forgiving and on valuing life could terminate me because I'm pregnant and am choosing to have this baby," said McCusker, who was fired last month. "I held the Catholic religion to a higher standard."

The issue might have gone away, she said bitterly, if she had ended the pregnancy -- thus violating another tenet of the Catholic faith.

"They wouldn't have known," she said. "I have been devastated by this entire incident. . . . This was my first teaching position and I was excited. I was looking forward to the school year with my young students."

McCusker had been teaching pre-kindergarten at St. Rose of Lima. After losing her $30,000-a-year job, she began working as a substitute teacher in New York City public schools. She is living with her parents on Long Island, and has declined to comment further.

The Brooklyn Archdiocese issued a brief statement, saying, "This is a difficult situation for every person involved, but the school had no choice but to follow the principles contained in the teachers' personnel handbook."

The handbook states that "a teacher is required to convey the teachings of the Catholic faith by his or her words and actions, demonstrating an acceptance of Gospel values and the Christian tradition."

The archdiocese's right to teach these values in its schools, and to expect teachers to uphold them in their personal lives, is inviolate, said Kiera McCaffrey, spokeswoman for the Catholic League, a national group.

"This issue won't stand up in court because it's a private matter involving a religious institution," McCaffrey said. "I think the New York Civil Liberties [Union] thought they might be able to intimidate us with this kind of action, but that's not going to happen."

If a male teacher at a Catholic school was found to have impregnated a woman he was not married to, McCaffrey said, the same code would apply.