The proposals were first outlined last month but in a speech in London today, Clegg is set to confirm the plans.
Under the new set of proposals, a mother will still be entitled to 52 weeks' maternity leave - three-quarters of it paid, but will be able to trigger flexible leave at any point after the first two weeks. This can be done by sharing the time, taking it in turns with her partner or taking time off together. The only rule will be that no more than 12 months can be taken in total, with no more than nine at guaranteed pay.
Clegg will say: "You won't get to 30 and suddenly have to choose motherhood or work because we're making the changes that will give you a route back."
He will add: "These are major reforms and at a time of continuing economic difficulty it is sensible to do them in a number of steps rather than one giant leap. More and more men are taking on childcare duties or want to and flexible leave builds on that."
Responding to the proposals, TUC general secretary, Brendan Barber, said: "The reforms announced today will make it easier for parents to choose how they want to share their leave to look after newborn children. It's great that parents who want to adopt will be given more support too.
"Allowing all staff to ask to work flexibly is common sense to good employers. But we know that too many businesses are still reluctant to modernise working practices so the government is right to give them a nudge with this new universal right to request flexible working."
Barber added: "These reforms will make life easier for millions of working parents. Businesses will also benefit from a more engaged workforce and a larger pool of people to recruit from."