Tens of thousands of women are forced out of their jobs each year for being pregnant, the Equal Opportunities Commission (EOC) has said.
These women miss out on £12m in statutory maternity pay, while employers spend £126m replacing them, the EOC added.
The EOC added that upwards of a million pregnant women will face workplace discrimination in the next five years.
Women must be better informed of their workplace rights, the group added.
"Pregnancy discrimination has a huge impact on their lives, but the harm it does to our economy affects us all," Jenny Watson, acting chair of the EOC, said.
"It's time for honesty about the scale of the problem. Employers - particularly small businesses - need more help in managing pregnancy at work."
Roughly 30,000 women a year lose their jobs because of pregnancy, but only 3% lodge a complaint at a tribunal, the EOC said.
About 200,000 pregnant women - or nearly half of those who work whilst pregnant - feel they have suffered some form of discrimination.
Ms Watson called for a package of measures to aid pregnant workers and employers including:
- A written statement of maternity rights and employer responsibilities, with a tear-off slip for the woman's employer
- Employers to have the right to ask pregnant employees to give a clear indication of when they intend to return to work
- Financial support for businesses
In response, Stephen Alambritis of the Federation of Small Business, welcomed the idea of employers being able to ask workers for a return-to-work date.
"What we would like to see is an acknowledgement that there are problems for smaller firms, and that we need to think imaginatively about how to help those small employers", he told BBC News.