More than a quarter of mothers in the UK feel discriminated against at work, a survey suggests.
A third of 1,975 women questioned for legal firm Slater and Gordon said they found it impossible to climb the career ladder and 54% said their employer could do more to support working mums.
Yet 35% thought they worked harder since having children.
Employers said businesses were better than ever at managing maternity leave and reintegrating mothers.
And England's employment minister Jo Swinson said it was 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," she added.
"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 from April 2015 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."
But a total of 35% of the mothers questioned across the UK by One Poll in July said their workplace was not supportive of their situation when they were pregnant and 31% felt they were not well treated by their employer while on maternity leave.
Some 27% said they had felt under pressure to return to work earlier than they wanted too.
Once back in the workplace, 29% felt they had been overlooked for a promotion because they had responsibilities as a mother.
In other findings:
- 48% thought their chances of career progression had been halted since becoming a mother
- 51% thought the attitudes of colleagues and bosses towards them changed once they had announced their pregnancy
- Of the 25% of mothers who felt they had been discriminated against at work, 48% felt overlooked for a promotion, 18% felt demoted and 35% had had responsibility taken off them
- 70% of mothers had never made a formal complaint about unfair treatment and of these, 26% did not want to "rock the boat"
Slater and Gordon employment lawyer Kiran Daurka said: "It is shocking that so few women speak out when they suffer discrimination.
"Employers aren't really talking about the issues here - they're burying their heads in the sand. They need to think creatively, because mums can work so flexibly.
"We need to get the issues out there. I hope this topic will become more openly discussed and, as women realise they are not alone, have rights and can take action, we can move forward."
But Neil Carberry, director of employment and skills at the CBI, said: "We don't recognise the picture painted here.
"Our experience is that the workplace has changed fundamentally over the past 30 years. Businesses are better than ever at managing maternity leave and reintegrating mothers on their return.
"The law is crystal clear on employers' responsibilities. It is important to be clear that no business should ever discriminate against a woman on the basis of pregnancy. If they do, she should feel confident to raise a grievance."
John Allan, national chairman of the Federation of Small Businesses, said: "Small businesses are family-friendly and often have a family-feel to them.
"Most will do all that they can to accommodate their staff during this period, allowing for changes to their working schedules and flexible working wherever possible.
"However, a member of staff going on maternity or paternity leave can have a significant impact on small businesses, so it is important that the employer is made aware of the pregnancy in a timely manner."
Siobhan Freegard, co-founder of the parenting website Netmums, said: "There is certainly a major problem with talented and experienced mums not being able to find jobs with family friendly hours, but the smartest companies are beginning to address this with ideas like flexible working, remote working and job shares.
"These companies know it's cheaper and more efficient to pay maternity leave and get back a committed employee who knows the business - rather than to make women so uncomfortable they leave and the firm is forced to expensively hire an unknown, untried and untested new member of staff."
Last month, former Labour minister Yvette Cooper revealed that she had felt "cut off" by Whitehall officials when she was on maternity leave for a second time.