There is a chance, as no form of birth control is 100% effective. I was put
on cerazette, a progesterone only pill 8 years ago as the combined pill gave me blackouts and migraines, and i took it every day between 8-9am. It completely stopped my periods straightaway as i started taking it after the last combined pill i took. Despite not having had a period in all this time, i fell pregnant last year, and my son is now 6 wee old.