What is the best statistical ysis to use in this situation?

I have to determine if there is a significant change in percentages from month to month from the implementation of a new procedure. The population size changes from month to month as well as the monthly calcuated percentages. How do I determine if the overall change in percentage from the end of the period is significantly different than the beginning? What statical tests can I run? I have calculated percentages from month to month as well as quarterly.