There are so many women who have doubt regarding observing Vrat during monthly menstrual periods. Such doubts arise when Ekadasi, Pradosh, Sankashti Chaturthi etc falls during the monthly period. It gets even more complicated when women observe Solah Somvar Vrat, Solah Shukravar Vrat etc.
The general rule is not to count the vrat day. Do observe a fast, but don't worship God and perform puja on that particular day. This is applicable when observing solah somavar vrat etc or every month vrat or every week Vrata.
That is if you have your periods during Solah Shukravar Vrat or other such vrats you have to do the vrat a day more. The vrat on the period day is not counted. (For example if it is a sixteen Monday vrat then it becomes a 17 Monday vrat as the one during the period is not counted.)
In case of Ekadasi, Pradosh, Sankashti etc you can perform the vrat but without doing pujas.
Another question is what if you get periods on the vrat day. The same rule is applied. Stop doing pujas etc but do the fast.