Hmmm, I had a horrible time with the birth control pills while fasting They caused me serious nauseau & vomiting as soon as the third day. At the time, my MD told me it was because the birth control pill needed something/anything in my stomach to break it down so it would be utilized with my body...and without food in there, it was trying to get what it needed from the lining of my stomach.
Now, that was YEARS ago and I've learned a lot since then (and The Pill has been changed in many ways), but I'd check around a lot before relying on The Pill for secure birth control before fasting.