Around 30,000 women a year are sacked, made redundant or leave their jobs due to pregnancy discrimination, according to research.
Some women said they had been under pressure to leave their jobs
The Equal Opportunities Commission (EOC) says half the 1,000 women questioned for its survey reported some level of bias against them.
EOC chair Julie Mellor said the findings were "shocking".
The organisation is asking the government to provide more support to both employees and employers.
"Although some employers knowingly flout the law, many businesses do face genuine challenges in managing pregnancy and simply don't know what their responsibilities are or what help is available to them," Ms Mellor said.
The EOC research indicated that 20% of respondents believed they lost out financially due to discrimination, and 5% said they were put under pressure to leave when they announced their pregnancy.
Sarah Holland fell pregnant when she had been working for a software development firm for almost five years, she told BBC One's Breakfast programme.
"I had been passing on e-mails to my partner about being excited about being pregnant and doing a test to confirm what we already knew," she said.
"We then had four days off together to celebrate. On coming back to the office that morning, I was immediately informed I was redundant.
"My boss said he had been thinking about it for the last 18 months, but he had no paperwork to back himself up at all. I was to work for three months to finish a project and then go."
EOC's Amanda Ariss told Breakfast: "Most employers want to get this right. They do want to support pregnant employees and help them come back to work."
She said there were "big benefits" to businesses that managed pregnancies successfully, pointing out that Nationwide saved around £3m a year by encouraging new mothers to return and avoiding the need to recruit replacements.
"There is a big problem with employers and employees understanding their rights and responsibilities," Ms Ariss said.
"That is why we are recommending that the government should give every pregnant woman a statement of her rights, including a very simple document to pass on to her employer explaining their rights and responsibilities.
"Many employers don't understand, for example, that they can claim statutory maternity pay back from the government."
She added that small businesses with fewer than 10 employees needed much more support.
"We think they need better information and they need more financial support, because they do face real challenges when they have pregnant members of staff."
There are approximately 441,000 pregnant women in the UK workforce each year, according to statistics.
The EOC says their survey is the first to measure the level of discrimination against them.
Women with jobs in the retail sector were most likely to face bias against them, with 53% of respondents working in that area reporting difficulties.
Those working in a financial environment were least likely to encounter discrimination, the survey found, with 42% saying they had problems.
Overall there was little difference in the rate of discrimination faced by women working in manual jobs and those in managerial and professional roles.
The survey results were released as the EOC unveiled a campaign, Pregnant and Productive, calling for action to end unlawful treatment of pregnant women at work.