If I'm reading your post correctly, you want to apply for a mortgage based on both salaries and you seem to indicate that you wouldn't get the amount you want on just your salary. But your wife is planning to leave work after her maternity leave. Is that correct? If so, I'd consider it a very risky proposition. If a bank won't give you the amount you want based on your salary alone, it's because they believe you can't afford that amount on just one salary. You need to consider the long term affordability, not just whether or not you can afford it now. Interest rates will rise - it's a question of when, not if. Tax rates may also rise. You may suffer a pay-cut.
I'd recommend that you lower your target and apply for a mortgage that you will be approved for, based on your salary alone.
That said, if you are applying for a mortgage now, there's nowhere on any mortgage application form that I've seen that asks you if you're pregnant.