New rights to allow parents to share leave following the birth or adoption of their child have become law.
The new rules mean that couples with babies due, or children matched or placed for adoption, after 5 April 2015 will be able to share leave.
After an initial two weeks, up to 50 weeks of leave and 37 weeks of pay can be shared.
However, one law firm said the change could reduce some businesses' level of pay to parents.
The government said that about 285,000 working couples would be eligible to share leave under the new rules, announced last year.
The rules mean that parents can also take time off at the same time to look after a newborn.
The pattern of leave must be agreed between the employee and employer with eight weeks' notice. Conciliation service Acas, which has drawn up a guide to the rules, said that employers and employees should be aware of the new rights.
However, the government has allowed individual employers to decide whether to extend enhanced maternity pay to all new parents taking shared parental leave.
Law firm Howard Kennedy said that said some smaller firms might decide to avoid risks of discrimination claims from fathers and reduce costs.
"We would not be surprised if some employers now retrench or withdraw enhanced maternity pay from new mothers rather than extending it to a broader category of employees, including new fathers," said Louise Gibson, of Howard Kennedy's employment law team.