This is extremely important, so please pay extra attention to this blog post.
Your pre-approval was given to you based on the amounts of money you had at the time when credit was pulled in your savings accounts, checking accounts, 401k accounts, other retirement accounts, etc. and was based on that amount of debt you had at that specific time. (any loans/credit cards, current mortgage if you have one, etc.) Do not go out and open any new accounts, or buy new furniture, car, TVs between now and closings - as this will put your final approval for the mortgage in jeopardy. THE LENDER PULLS YOUR CREDIT AGAIN RIGHT BEFORE CLOSING... You need to minimize your expenses and spending between now and your closing.
Pay all your monthly bills on time, don't open or close any credit accounts or credit cards, don't go on big vacations that would change your balances in those accounts above, etc. I have had clients buy a new couch on credit, and then we have to figure out a different game plan because they now cannot qualify for the loan the way it was set up - up to not being able to qualify at all. Once closing happens and settlement, you can go wild, but don't buy Anything big until after settlement.
If you have an expense you have to fulfill, please talk to your mortgage lender you selected and they will be able to tell you what you can/cannot do.