It is the estrogen in other types of birth control pills that decreases milk supply and thus makes nursing more difficult. (breastfeeding.lifetips.com)
In fact, it's actually the preferred method for people who can not take other types of birth control due to complications. (mindbodygreen.com)
There are certain types of birth control pills the doctor can prescribe that are safer for nursing mothers and please note that certain hormones in birth control pills can actually suppress your milk supply. (newkidscenter.com)