A study by law firm Slater & Gordon showed some employers were still penalising women for getting pregnant.
The firm questioned 2,000 mothers on how they were treated before and after the birth of their child.
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, and 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.
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 found that most did not make a formal complaint about the discrimination.
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."
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