One day in October 2004, Tanys Lancaster says, she met with her manager at Bloomberg L.P., the media and financial-data empire founded by the mayor of New York City, Michael Bloomberg. She’d been demoted, her boss informed her, and her bonus was being slashed by a third. “Accept it,” she recalls her manager telling her.

It was not the sort of directive that Lancaster, now 39, who’d been with the company for 10 years, was likely to accept without protest.

“I was livid,” she says today. She was also feeling sick and miserable. Lancaster had undergone months of in vitro fertilization treatments and was pregnant with twins—news she’d shared with her supervisor as soon as she found out, the month before.

At the time of her demotion, Lancaster was one of Bloomberg’s highest-ranking female executives, managing a team that sold the company’s financial software worldwide. Her compensation was generous—a $130,000 base salary and a $285,000 deferred bonus the previous year. “It was a phenomenal place to learn and develop,” says Lancaster, a blond, green-eyed Englishwoman. She sits tensely in her lawyer’s downtown Manhattan office as she speaks for the first time to a reporter. “I worked with motivated, professional people. We worked very hard, and we had high-profile customers. It was exciting and challenging.”

Following her demotion, though, and especially after she returned from maternity leave, Lancaster says, her status within the company went downhill. Time spent caring for her newborn twin boys cut into the marathon hours she used to put in at the office. She found herself without an official department and dropped another rung on the ladder until finally she complained to human resources, claiming that she hadn’t assumed an equivalent position in the company upon returning from maternity leave, as is required by federal law.

Nothing changed. Almost a year to the day after she was demoted, Lancaster resigned, convinced that she’d been pushed out because she had become a mother. She hired a lawyer, who filed a complaint with the federal Equal Employment Opportunity Commission, which enforces antidiscrimination laws in the workplace.

The E.E.O.C., it turned out, had already fielded a similar charge against Bloomberg L.P. on behalf of another employee, Jill Patricot, and would soon begin looking into a third. In September 2007, the agency filed a class-action lawsuit against the company on behalf of Lancaster, Patricot, and Janet Loures, all of whom worked on the business side, plus a group of women who worked at the company between 2002 and the present. The plaintiffs had all, in the view of the E.E.O.C., suffered discrimination. They now total 72, out of about 500 women who took maternity leave during that time, a high percentage, according to the agency. (Many are still employed at the company, including Patricot and Loures; Lancaster is at Thomson Reuters.) The named plaintiffs say their pay was cut and their job responsibilities reduced once they became pregnant and also after they returned from maternity leave. In a 45-page response, Bloomberg L.P. denied the allegations, as well as Lancaster’s version of the events that led to her resignation. Company spokeswoman Judith Czelusniak said, “We intend to litigate this case in the court, not the press, and we are confident that once all the facts come out they will demonstrate that the claims have no merit.”

The first of dozens of depositions in the E.E.O.C. case was taken in October, and the agency’s lawyers have notified Mayor Bloomberg that they intend to take his deposition as well. The class-action lawsuit, coinciding with Bloomberg’s campaign for a third term as New York’s mayor, might be expected to complicate his bid.

So far, however, it has not. His company’s legal problems are a subject that Bloomberg, 66, is notoriously sensitive about, and he seldom discusses them, although he owns almost 90 percent of the company. (He declined to comment for this article.) Since taking office in 2002, he has officially recused himself from most of his firm’s day-to-day affairs. But periodically a question comes up about his precise level of involvement, as it did when the E.E.O.C. lawsuit was first filed. Bloomberg initially denied knowing anything about the suit; then, a week later, he revealed that he’d talked about it with his company’s management. “I am the majority owner, and I’m absolutely entitled to talk to the senior people and am entitled to know what’s going on,” he said.