Hundreds of women in Northern Ireland claim their career opportunities have been hampered because of pregnancy or maternity leave.
An investigation by the Equality Commission, which monitors compliance with equality legislation in the region, reported unfair treatment such as the termination of employment, a change of role and losing out on pay increases.
Equality Commission chief executive Dr Evelyn Collins said the situation was unacceptable.
She said: "Over one-third of the women who talked to the Commission about their experiences said that they had been treated unfairly or disadvantaged because of their pregnancy or because they took maternity leave.
They believe this affected their finances, their career opportunities, their status at work and their health. This is not acceptable 40 years after the introduction of legislation in Northern Ireland to provide protection from sex discrimination in employment."
More than 900 mothers working in the private and public sector, with children aged under five, took part in the research.
A significant percentage, 36%, said they believed they had been treated unfairly or disadvantaged as a result of their pregnancy or maternity leave.
They believed that as a female employee they were expected to make a choice between parenthood and a rewarding career path.
Some women reported firms' failure to consider the risks to health and safety of pregnant employees.
Others said they were overlooked for promotion; a dilution of work responsibilities; were denied training; had changes to working hours, experienced a reduction in bonus payments and were subjected to negative comments.
The findings of the report - Expecting Equality: A Formal Investigation Under the Sex Discrimination (Northern Ireland) Order 1976 - will be presented at a conference in the Titanic Belfast today.
Employers were invited to share their experiences. Of those surveyed, most said that they provide support for pregnant employees and new mothers, referring to policies they had in place including flexible working arrangements, childcare vouchers and return to work incentives such as phased return and bonus payments.
Dr Collins said it was encouraging that almost half the women who responded thought their employer had been supportive during their pregnancy, and on their return to work.