More than one in four mothers feel they have been discriminated against at work while pregnant or after returning to their job, according to new research.
Two thirds also said they would advise women to wait until the last possible moment to tell their bosses they are expecting.
But the survey of almost 2,000 women found that most did not make a formal complaint about the discrimination.
The study, by law firm Slater & Gordon, questioned mothers on how they were treated before and after the birth of their child and showed some employers were still penalising women for getting pregnant.
Almost a third said they were not treated well during their pregnancy and maternity leave, while almost half were overlooked for a promotion, almost a fifth demoted, while more than a third had responsibility taken off them.
Two out of five also said younger colleagues without children were given more support and encouragement.
Kiran Daurka from Slater & Gordon said: "Despite the equality legislation in place, attitudes and working practices continue to block women in achieving their career aspirations in the UK.
"Anecdotally, we hear of mothers complaining about being put on a 'mummy track' when back at work, and this research illustrates that this is a real experience for many women."
It is illegal to sack a women because she is pregnant or on maternity leave, said Employment Minister Jo Swinson.
She said the Government was committed to supporting women's talents and that shared parental leave and pay, being introduced in 2015, "will allow couples to choose how they share care for their child in the first year after birth".
However, TUC general secretary Frances O'Grady said newly-introduced fees for employment tribunals would deter some women from tackling discrimination.
"Sadly some employers are still living in the dark ages when it comes to women in the workplace," said Ms O'Grady.
"[But] by introducing tribunal fees of up to £1,200 to take an employer to court over pregnancy discrimination, the Government has ensured that many more of these women will have to suffer in silence."