Democratic Presidential hopeful US Senator from Massachusetts Elizabeth Warren speaks during the SEIU Unions for All Summit in Los Angeles, California on October 4, 2019.Frederic J. Brown | AFP | Getty ImagesDemocratic presidential candidate Elizabeth Warren stuck to her story Tuesday about losing her first teaching job because of pregnancy amid new scrutiny of the narrative.Recent reports have cast doubts on the story, which the senator from Massachusetts has highlighted on the campaign trail as a key part of her background. In a pair of tweets Tuesday, Warren — one of the leading candidates aiming to challenge President Donald Trump for the White House next year — stood by her account of how she lost the position.”When I was 22 and finishing my first year of teaching, I had an experience millions of women will recognize. By June I was visibly pregnant—and the principal told me the job I’d already been promised for the next year would go to someone else,” wrote Warren, who is 70.She continued: “This was 1971, years before Congress outlawed pregnancy discrimination—but we know it still happens in subtle and not-so-subtle ways. We can fight back by telling our stories. I tell mine on the campaign trail, and I hope to hear yours.”Warren has run for the White House as a champion for both labor and women’s issues, arguing for systemic change to boost workers. The story about losing her teaching job falls into a sequence of events that Warren says helped to push her toward law school and eventually the Senate.A recent surge in national polls has Warren jockeying for position with former Vice President Joe Biden at the front of the Democratic pack.Several reports have put new skepticism on Warren’s story. In a 2007 interview at the University of California at Berkeley, she did not specifically say the New Jersey school where she worked hired a different teacher because of her pregnancy.I was married at nineteen and graduated from college after I’d married, and my first year post-graduation 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. 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, and I was really casting about, thinking, “What am I going to do?”A Washington Free Beacon story published Monday also raised questions about Warren’s story. The news outlet writes that the Riverdale Board of Education in New Jersey approved Warren for a second year of teaching in a two day per week job, according to school board meeting minutes.Two months later, the minutes show Warren’s resignation was “accepted with regret,” according to the Free Beacon report.Before she tweeted Tuesday morning, Warren first defended her story in an interview with CBS News on Monday. She said, “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.”In a statement to CBS, the senator also explained the differences between how she described her departure from teaching in 2007 and the story she told when she became a more public figure.”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.Subscribe to CNBC on YouTube.