Hello Eloise,
You can create segments with data ranges (last week, thisweek) then you can use this to build calculateds metrics (this week ÷ last week). In the workspace you can use this in dimensions that not include dates. (It will always displays the currently week and the last week)
I recommend you to use report builder, it's more flexible to compare any time period (week 13 vs 12, then 12 to 11).