On the campaign trail, Elizabeth Warren often tells the story of how she was fired from her first teaching job in 1971 because she was pregnant, a pivotal moment that ultimately put her on a path to Harvard, the United States Senate, and quite possibly the presidency. But recently, several media outlets have questioned the veracity of these claims.

In an exclusive interview with CBS News on Monday evening, Warren said she stands by her characterizations of why she left the job.

"All I know is I was 22 years old, I was 6 months pregnant, and the job that I had been promised for the next year was going to someone else. The principal said they were going to hire someone else for my job," she said.

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Warren has repeatedly said that her principal "showed [her] the door" after discovering she was pregnant at the end of the 1971 school year. The episode is pivotal to her life story, in that it dashed her dreams of remaining a public school teacher and launched her reluctantly down the path to public service.

Fresh out of the University of Houston, Warren was hired by the Riverdale Board of Education in New Jersey as a speech pathologist for the 1970-1971 school year. Since she began her campaign for the presidency, she has repeatedly said that she was "shown the door" after just a year as a result of her pregnancy.

"By the end of the first year I was visibly pregnant, and the principal did what principals did in those days: wished me luck, showed me the door, and hired someone else for the job," she said at a town hall in Oakland in June.

The "showed me the door" anecdote came up often on the campaign trail until recently. And now some outlets have found a 2007 interview Warren gave in which she presents the story in a different light.

In an interview that year at the University of California, Berkeley, Warren gave the first known public account of her time at Riverdale.

"I worked in a public school system with the children with disabilities. I did that for a year, and then that summer I didn't have the education courses, so I was on an 'emergency certificate,' it was called," Warren said in 2007. "I went back to graduate school and took a couple of courses in education and said, 'I don't think this is going to work out for me.' I was pregnant with my first baby, so I had a baby and stayed home for a couple of years."

Asked by CBS News why she told the story differently at Berkeley a decade ago, Warren said her life since her election to the Senate in 2012 caused her to "open up" about her past. "After becoming a public figure I opened up more about different pieces in my life and this was one of them. I wrote about it in my book when I became a U.S. Senator," she said in a statement from her campaign.

Warren's changes in phrasing when discussing her dismissal have sparked questions about her story's veracity. Fox News has cited the 2007 interview as a contradiction with her more recent statements. The Washington Free Beacon reported on a transcript from contemporaneous local school board meetings, also obtained by CBS News, which said Warren was rehired that spring and that the board "accepted with regret" her "resignation" the following summer.

In fact, the school board minutes show that the board voted by unanimous roll call to extend Warren a "provisional certificate" in speech pathology.

Local newspaper reports from 1971 also present reasons for her leaving the school alternative to what she describes on the trail. The Paterson News, a local paper, reported that summer that Warren was "leaving to raise a family." The next month, a story about the school board hiring a replacement said Warren had "resigned for personal reasons," even though the board had voted to "appoint" Warren to the same speech pathology job that April, according to an earlier report.

Warren told CBS News she stands by her characterization of getting "shown the door" because of her pregnancy and called it an "accurate description."

"When someone calls you in and says the job that you've been hired for for the next year is no longer yours. 'We're giving it to someone else,' I think that's being shown the door," Warren told CBS News.

In her 2014 memoir, published after she became a Massachusetts senator, Warren gave a similar account of her departure from Riverdale Elementary.

Warren also told CBS News that she was, in fact, officially offered the job for the following year as the school board minutes indicate. "In April of that year, my contract was renewed to teach again for the next year," Warren said. She also said she had been hiding her pregnancy from the school.

"I was pregnant, but nobody knew it. And then a couple of months later when I was six months pregnant and it was pretty obvious, the principal called me in, wished me luck, and said he was going to hire someone else for the job," Warren said.

Asked repeatedly whether she meant she was fired when she said the principal showed her the door, Warren said, "When someone calls you in and says, the job that you've been hired for for next year, is no longer yours, we're giving it to someone else. I think that's being 'shown the door.'"

Recent profiles of Warren have referenced her departure from Riverdale Elementary and that stated she was fired from her job. "Warren was laid off when she became pregnant, and after her daughter was born," The New York Times Magazine reported in June. "She called the role a dream job, and one that she still might have today had she not been terminated for being visibly pregnant," The Huffington Post reported a month before.

Later in the summer, however, Warren was telling the same story about the principal without the part about him showing her the door.

"I was visibly pregnant. And back in the day, that meant that the principal said to me," Warren said, pausing, "wished me luck and hired someone else for the job.

Asked about this change in stump speech, Warren told CBS News she "actually hadn't noticed" she does not reference her firing like this anymore.

Interviews with retired teachers who worked for the Riverdale Board of Education at the same time as Warren suggest that while they do not remember Warren or the circumstances of her leaving the school, the workplace culture at the time may have left Warren with no option but to move on when her pregnancy became apparent.

Two retired teachers who worked at Riverdale Elementary for over 30 years, including the year Warren was there, told CBS News that they don't remember anyone being explicitly fired due to pregnancy during their time at the school. But Trudy Randall and Sharon Ercalano each said that a non-tenured, pregnant employee like Warren would have had little job security at Riverdale in 1971, seven years before the Pregnancy Discrimination Act was passed.

"The rule was at five months you had to leave when you were pregnant. Now, if you didn't tell anybody you were pregnant, and they didn't know, you could fudge it and try to stay on a little bit longer," Randall said. "But they kind of wanted you out if you were pregnant."

As the school board minutes show, no member of the Riverdale school board at the time was a woman. A full year after Warren's dismissal, the Associated Press wrote that a recent New Jersey State Division of Civil Rights decision meant that "pregnant teachers can no longer be automatically forced out of New Jersey classrooms."

After leaving her position at Riverdale Elementary, Warren took education and accounting courses at local New Jersey colleges before applying to Rutgers Law School in 1973, according to her law school application. She graduated in 1976, again "visibly pregnant," and became a lecturer in law at Rutgers in 1977.

By the early 2000s, she had become a leading public intellectual and an expert on bankruptcy law. She later was the architect of the Consumer Financial Protection Bureau and was elected to the U.S. Senate from Massachusetts in 2012. But according to Warren, had she not been pushed out of that teaching job at Riverdale Elementary, she would likely still be teaching special needs students.

"I would probably still be doing it today, but life has a way of putting another kink in it. By the end of the first year, I was visibly pregnant," Warren said at the Manchester Democratic City Committee's Flag Day dinner in June.

"And, back in the day, the principal did what principals did. Wished me good luck, showed me the door and hired someone else for the job."