By Paul Lewis
BBC Radio 4's Money Box
The laws covering maternity rights are too complicated, the TUC has told the BBC.
Maternity leave and pay are to be extended from Sunday
As rights for new mothers change again on Sunday, the organisation blamed the complexities for many mothers and employers not understanding the rules.
It called on the government to do more to tell women of working age their rights if they become pregnant.
And it warned that employers are breaking the law due to ignorance of maternity rules.
The TUC was responding to surveys from the Equal Opportunities Commission and the Institute of Payroll Professionals.
The studies indicate that at least half of mothers and many employers are ignorant of existing maternity rights and the changes that begin on 1 April.
Under the new rules, working mothers whose babies are expected on Sunday, 1 April 2007 or later, will now qualify for a full year's maternity leave - however long they have been with their employer.
Previously, some employees who had worked for less than nine months got just six months leave.
Women who earn at least £87 a week and have been at their place of work for 41 weeks before the baby is due can now get 39 weeks maternity pay - six weeks at 90% of their full pay, and another 33 at up to £112.75 a week.
Sarah Veale, head of equality and employment rights at the TUC, told BBC Radio 4's Money Box that these different rules made it hard for employers to know - or explain - what mothers' new rights were.
"There are two different systems which we find ridiculous," she said.
"One is a benefit from the DWP and the other an employment right so comes under the auspices of the DTI.
"I'm not surprised employers get into a muddle. The two things have different qualifying periods and cover different amounts of time."
And she had a warning for small employers: "They have to accept that just because they are small doesn't mean they have less responsibility to women than large employers. But I can see a strong argument for the Revenue taking over payment of maternity pay and perhaps giving more help assisting small firms with getting temporary labour in and handling absences," she said.
She supported the other new right to 10 "keeping-in-touch" days while women are on maternity leave.
"She can come into the workplace for up to 10 days," she said. "It cannot be more than 10 and the employer cannot force her to do it.
"The idea is to spend time meeting new colleagues, learning new systems."
But she warned: "She is not supposed to be there to do a day's work. If someone was asked to come in because the temp hadn't turned up she would be absolutely right to refuse to do that."
BBC Radio 4's Money Box was broadcast on Saturday, 31 March 2007 at 1204 GMT.
The programme was repeated on Sunday, 1 April 2007 at 2102 GMT.