You need to learn to budget.
Big sheet of paper, or an Excel spreadsheet, all your bank statements for the last few months.
now work out what you spends on essential things, mortgage, council tax, bills, insurance, cars,
then things that are necessary but variable like food and petrol and essential clothes and shoes, and often these can be cut down a lot
then the non essentials of entertainment, takeaways, tv subscriptions, music subscriptions, clothes and shoes bought just because, coffees and meals out, bought lunches at work and so on
then work out your family income which will be wages, maternity pay, child benefit and see how you compare. More income than essentials , you will be fine, less income than essentials, work out the difference and save that money to cover each month of mat leave