Employers are being accused of having "antiquated" attitudes to recruiting women as new evidence revealed many believe it is reasonable to ask about plans to have children during a job interview.
A survey of over 1,100 decision-makers found that three out of five agreed a woman should have to disclose whether she is pregnant during the recruitment process and almost half thought it was ok to find out if women had young children.
The Equality and Human Rights Commission (EHRC) said its study showed that many employers needed more support to better understand the basics of discrimination law and the rights of pregnant women and new mothers.
EHRC chief executive Rebecca Hilsenrath said: "It is a depressing reality that, when it comes the rights of pregnant woman and new mothers in the workplace, we are still living in the dark ages.
"We should all know very well that it is against the law not to appoint a woman because she is pregnant or might become pregnant.
"Yet we also know women routinely get asked questions around family planning in interviews."
Sarah, a mother of two young children who was made redundant during maternity leave for her first child, said: "It's sad to think that things like this are still happening. I feel angry all the time that you can be a mother with young children and unless you're in a job that protects you, your whole world can come tumbling down - out of your control.
"It is essential for employers to be honest and ensure there is good communication between them and those on maternity leave so that pregnant women and new mothers are given the support they deserve."
The EHRC said its survey revealed antiquated beliefs, including two out of five saying women who have had more than one child while in the same job can be a "burden" to their team.
Half of those questioned said workers sometimes resented women who were pregnant or on maternity leave.
TUC general secretary Frances O'Grady said that women should not be forced to choose between having a career and a family.
"But thousands are being forced from their job every year. Pregnancy discrimination scars lives and careers.
"Employers are getting away with breaking the law on an industrial scale."
Young Women's Trust chief executive Dr Carole Easton said employers and the economy were both missing out on the talents of young women.
"Today's findings show many employers, given half a chance, would run roughshod over women's rights.
"It's no wonder women are held back in the workplace when people have such outdated, discriminatory views.
"It is employers and our economy that miss out on the talents of young women as a result. Young women who want to work are, meanwhile, finding themselves in debt and relying on food banks."