WASHINGTON — It's a staple in her campaign stump speech: Sen. Elizabeth Warren, D-Mass., explains how she was fired from her first teaching job in Riverdale, New Jersey, because she was pregnant.

But in recent days, Warren has been the target of a largely conservative media-driven assault on this aspect of her personal narrative. Citing documents from the 1970s — when Warren worked at Riverdale Elementary — and a 2007 interview in which Warren did not mention her pregnancy as a reason for leaving her job, outlets like the Washington Free Beacon have sought to poke holes in her account. But the senator and presidential candidate is standing firm.

"When I was 22 and finishing my first year of teaching, I had an experience millions of women will recognize," Warren tweeted Tuesday morning. "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."

Warren followed that tweet by encouraging women who also experienced gender discrimination to "fight by telling our stories. I tell mine on the campaign trail, and I hope to hear yours."

Minutes from several Riverdale Board of Education meetings, reviewed by NBC News, show that Warren was approved to be back at Riverdale Elementary School part-time — until she wasn’t. The documents, from April 21, 1971, show Warren was issued a two-day-a-week speech contract for her second year of teaching — a similar job to what she’d been doing the previous year. Warren would have been at that time around four months pregnant, and she told CBS News in an interview Monday night that she hadn't told anyone she was pregnant yet.

Two months later, minutes from a board meeting on June 16, 1971, say Warren's resignation was "accepted with regret." At that point, Warren would have been about six months pregnant — and likely showing. Her daughter was born Sept. 2, 1971.

"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.

Two former Riverdale teachers told CBS News that, while they do not recall Warren's specific situation, it would have been standard practice at the time.

"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," one of them, Trudy Randall, told the outlet. "But they kind of wanted you out if you were pregnant."

Congress did not pass the Pregnancy Discrimination Act until 1978, and it was historically not uncommon for women to experience job discrimination for being pregnant. And as a New York Times investigation from earlier this year reported, pregnancy discrimination remains systemic to this day.

The other discrepancy stems from a 2007 talk at the University of California at Berkeley, in which Warren described leaving her first teaching job without mentioning her pregnancy as the direct reason.

She explained, "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."

Asked about the 2007 speech, Warren told CBS that her election to the U.S. Senate in 2012 caused her to "open up" more about her past.

The increased scrutiny of Warren's personal story comes after a summer of rising in the polls and solidifying herself as one of the top candidates in the 2020 Democratic primary field.