Delaying what you know to be the inevitable will only reduce the options your lender will have for you. As soon as you know you won't be able to pay, call!

Failure to make your mortgage payment will result in your lender reporting the mortgage lates to the credit reporting bureaus. Your initial late payment may result in a 30 day late which can bring your FICO scores down. If you do not clear the delinquent payment, then 60 day, 90 day, and 120 day lates may be reported as well which can lead to possible foreclosure of your home.

Too often people who are unable to make their mortgage payment wait until its too late to contact their current mortgage lender. You do not want to wait until you have fallen 2, 3, 4, or even 5 months behind on your mortgage to contact your lender.
You are not the first person to be in this situation and it is in both your and your lender's best interests to work something out that will suit the both of you.