Maternity discrimination is a growing problem in the workplace, with employers' attitudes "hardening" against pregnant women, according to new research.
A charity said almost 10% of calls to an advice line related to maternity issues, revealing that discrimination was "rife".
Working Families said its study also showed that some employers were "unpicking" flexible working arrangements, with little consideration of the impact on family life.
Cases dealt with by the charity included a cleaner with four years service who was off for a week with a pregnancy-related illness and returned to work to find her name had been left off a work rota. She was told to take three months maternity leave or lose her job.
Other examples included a woman told to start working on Saturdays despite telling her employer she had to look after her child, and a female worker who was told her pay would be cut if her request to work flexibly was accepted.
Sarah Jackson, chief executive of Working Families, said: "This is the third year we've reported on high levels of maternity discrimination, with signs that employer attitudes are hardening and discrimination becoming more blatant. Eight years ago - before the recession hit - the Equal Opportunities Commission found 30,000 women lost their jobs each year because of pregnancy or maternity. It is time the the issue was revisited, as we believe our helpline reveals only the tip of the iceberg.
"We're also calling on employers to recognise the impact of their imposed changes to contract on families already struggling to balance work and care. Parents can't suddenly change their hours to work Saturdays or evenings if childcare isn't in place. We call on the Government to use the forthcoming Budget to provide extra help with childcare costs and protect families from further cuts. Our helpline reveals that parents are struggling to make work pay."
Some women who contacted the charity were reluctant to take any action for fear of losing their jobs, said the report.
Employment minister Jo Swinson said: "It is illegal to sack a woman because she is pregnant or on maternity leave. Such action constitutes pregnancy discrimination and could result in an employer in front of an employment tribunal. The Government is committed to making sure that more businesses make the best use of women's talents throughout the organisation, from boardroom to the shop floor. This is why we're introducing shared parental leave and pay which will allow couples to choose how they share care for their child in the first year after birth.
"The new system will shatter the perception that it is mainly a woman's role to stay at home and look after the child. It will also allow women to choose to return to work earlier if they wish, reducing the impact of pregnancy on women's careers. The Government is also extending the right to request flexible working to all employees."