Hi D.. While you're nursing, your body produces a hormone called prolactin that suppresses ovulation. Once you stop nursing, or start nursing less frequently, the level of prolactin in your system decreases. Progesterone-only pills do not interfere significantly with your lactation, however estrogen pills ("normal" hormone birth control pills), even the low-dose ones, do interfere and can have a significant impact on your milk supply. I took a progesterone-only ("mini") pill while I nursed, and had no problem (and there's nothing wrong with my daughter). I switched back to regular birth control about two weeks after she stopped nursing, and within two weeks I had no milk left (I was still trying to pump 3-4 times a day). Just FYI, it can take your body up to 6 months after you stop nursing and pumping to get rid of all the prolactin; until all that prolactin is either gone of significantly diminished you won't ovulate (per my gynecologist).