A married or civil partnership couple can claim Marriage Allowance when one member of the couple does not pay income tax and the other partner pays basic rate tax. 

When you are on maternity leave, your yearly pay may be less than your income tax personal allowance. It may mean that you do not need to pay tax for two tax years: the tax year of your pregnancy and the tax year you return to work. If your spouse pays tax at the basic rate you can claim marriage allowance at: https://www.gov.uk/apply-marriage-allowance

Marriage Allowance is back dateable to the April 2015 and can be as much as £250 a year which is a nice windfall when you’ve just had a baby.