Weeks are funny, ill-defined thing. Here's everything everyone can agree on about them--they span exactly 7 days. That's it. You can usually build a consenus about them starting on Sunday, but that's not a hard-coded fact. When you start talking about weeks, its a fuzzy issue.
Then when you start numbering them, it gets even more ill-defined. There's no set standard for doing so.
You need to explain how you number your weeks, when your week starts and how you would determine the number of your current week. If you could post sample data from your table, including field and table names that would be helpful too.