Briony, someone posted awhile ago about breakthrough bleeding causes (Hurdity?). If I recall correctly, it could go either way. Too little estrogen would perhaps not supress ovulation and you could have random spotting. On the other hand, too much estrogen could cause the lining to grow to a point that it kind of atrophies (maybe not the right term in this case) and breaks up a little on its own, leading to spotting. I know in my personal experience, my body was too particular for any pill. I tried several different ones in my twenties for 3-6 months at a time and had spotting/breakthrough with all of them. There was one I did fairly well on, but if I was even an hour late taking it, I would bleed. The doctor didn't trust that it was giving sufficient contraceptive protection so I had to abandon hormonal contraceptives altogether.