By Ian Dunt
Nick Clegg will unveil revolutionary plans later which would allow couples to share their parental leave.
Under the reform, mothers would be able to return to work after two weeks if they chose and fathers could use some of their leave to stay at home.
"So many couples feel like they are facing an impossible mathematical equation," Clegg will say later.
"And it is an equation where the answer is almost always rigged. Because whichever way you look at it, the solution ends up with the mother doing more of the caring and the father doing more of the earning."
Under the plans, a new mother can trigger flexible leave after two weeks. Parents can then share the remaining 50 weeks between them as they like – either in turns or at the same time.
Maximum leave will stay at 12 months, nine of them on guaranteed pay.
Paternity leave will remain at two weeks, although there will be a review in 2018, when the economy is expected to be stronger.
"I have accepted that extending paternity leave should be revisited when the economy is in a stronger state," the deputy prime minister will say.
Under plans in place since April, parents can share some of the parental leave, with the father free to use six months once the baby is 20-weeks-old. But the leave can only be taken in one block, like the mother's.
"It's not for us in government to tell you or anybody how to divide up your time between mums and dads when people have children," Clegg said in an interview on the Daybreak programme today.
The prime minister will also use the speech to pledge a right to flexible working in law for all employees.