What are the chances of being pregnant if you are on birth control and have just had your period?

This depends on how long ago you had your period. If there is fear, purchase a test if you are late. Speaking from personal experience.....I took bcp everyday around the same time and still got pregnant with my first child (then again many that was on that pill got pregnant). BCP are only 99%, so to be safe...purchase a test. One reason is you don't want to continue with the BCP if you are pregnant and you will need to see an OBGYN.