We have to use your last 2 months' income at the time you apply, regardless of whether this is reduced income due to maternity/paternity leave. This is because we can't know how much longer you'll be off for, or if your circumstances might change.
For example, you might decide to return to a different role or reduce your hours, which would mean reduced pay. Unfortunately, we can't extend the amount of time you have to provide documents, as the credit data for your application is only valid for a limited period.