it is EXTREMELY difficult to get pregnant according to all the studies I've read and there have been quite a few since my job is contraceptive research, there are things that can affect the effectivity of the pill though, such as st. johns wart and other herbal substances along with antibiotics, some pills don't work for everyone and espcially the only one hormone pill or POPs
For best results take a combo hormone pill such as yasmin or yaz or orthotricycline (etc), also follow directions and to be most safe don't have unprotected sex unitl you've been taking the pill for a full 28 days (one full cycle)
you can also take a few ovulation tests too see if you ovulate while on the pill because if ya don't ovulate getting prego is impossible
hope it helps