I think you can only tell them after the change has happened, so when you start maternity leave you can let them know, then they disregard the first £100 per week of smp or maternity allowance (whichever you claim) Then call them again as soon as you've had the baby and they'll adjust again to cover your new one!
phone them now and tell them that your situation will change shortly - they'll probably tell you to phone them when you go on maternity leave but it may take them a couple of weeks to sort it out and you don't want to have to worry about money at a time like that. I phoned them after putting in my first ever claim for tax credits at the end of last year as the forms asked for the previous years earnings which were clearly not relevant by the time i went on maternity leave and receiving only SMP. They were very very helpful on the phone and just updated the information and said the recalculation would be sent shortly. I received it few days later and the money was already in the bank. So don't worry - they'll sort it based on your estimated earnings x
I'm surprised you get as much as £50 a week based on this household income. Do you have children to support?
The best way to find an answer ti your question is to ring them- they probably have more details of your case than we do