With the current hormone mixtures most oral contraceptive pills (OCPs) have a woman has a period every month (during the 7 days she either takes no pills or takes placebos) to shed the uterine lining.
There are newer OCPs and devices, called continuous-use birth control, that women can take for 84 days - 1 year (or 2 yrs for an IUD or vaginal ring, or indefinitely for a Depo-Provera injection - with the caveat that a woman may never have a period again) before stopping for one week to shed the uterine lining.
Because these are fairly new to the market there is zero longitudinal data on the safety of these products. And as stated above, long-term use can result in never having a period again, ie: never being able to conceive.