The research of 2,000 working mothers by law firm Slater and Gordon found many mothers felt employers' attitudes towards them changed once they fell pregnant.
Half of those polled said they felt left out or not taken seriously at work after having a child, while two in five believed younger colleagues with no children were given more support and encouragement.
Three in five said they thought pregnancy was a problem in their workplace and a third said they found it "impossible" to climb the career ladder after giving birth.
The research found almost half of working mothers felt having children halted their career progression and four in 10 felt they don't have sufficient support from their managers.
Undervalued at work
The research, which polled mothers from sectors including health, education and legal, found many felt frustrated when returning to work, while a fifth said they felt less valued in the workplace.
According to the findings, the most common problems mothers face include colleagues' frustration at their part-time hours, not being included socially or in business-related matters, and a perception that their role is just a job now rather than a career.
Slater and Gordon lawyer Kiran Daurka said despite equality legislation, attitudes and workplace practices continue to block women's careers.
"I find it quite dispiriting to hear that more than a fifth of mums feel that they need to prove themselves to their bosses following their return from having a baby," said Daurka
This kind of attitude is short-sighted and bad for business," said Daurka.
"It is in everyone's interest to ensure that working mothers are allowed to work to their full potential."
TUC general secretary Frances O'Grady said: "Pregnancy discrimination causes terrible suffering for women and their families.
"No modern business should prevent staff pursuing their career just because they've become parents, but sadly some employers are still living in the dark ages when it comes to women in the workplace."
Earlier this year the Government introduced shared parental leave and pay. From April 2015 couples can choose how they share childcare in the first year after birth.
