if you're taking the pill properly, you shouldnt get pregnant by rights.
wikipedia said:
The combined Pill prevents pregnancy primarily by preventing ovulation. It also has the side-effect of thickening the mucous over the cervix, which can prevent or slow sperm entry into the uterus. The Pill also thins the endometrium (the lining of the uterus).
There are physicians who point to this thinning of the endometrium as evidence that The Pill is an abortofacient (causes abortions). This claim is based on experiences with in vitro fertilisation which demonstrated that thinner uterine linings correlated with increased difficulty in getting the test-tube-fertilized eggs to implant.
so methinks that even if you did stuff something up (eg. forgetting lots), and therefore fell pregnant, your body would stop it from getting any further when it started getting the chemicals again (probably a day or so after you started taking them on time, etc)