Q&A: Is Birth Control Safe?
Yes. However, “regular” birth control pills (a combo of estrogen and progestin) will hurt the quantity and quality of your milk supply. It's better to go with a progestin-only pill (aka “the mini pill”). There is no evidence that the progestin-only pill has any negative effect on baby, but the hormone will enter your milk.
IMPORTANT: You'll need to take the mini-pill at the EXACT SAME TIME every day to be protected. If you skip a pill, you might get pregnant. Set the alarm on your cell phone, or plan to take the pill at a super-obvious time each day (like when you get in the car to go to work). The mini-pill is slightly (1 percent) less effective than traditional birth control pills, assuming you are taking it according to the instructions. Luckily, exclusive breastfeeding keeps most moms from ovulating for several months, so you'll be doubly protected. (But don't use breastfeeding as a form of contraception. Women do sometimes get pregnant while exclusively breastfeeding, even if they haven't had a postpartum period.)
Your other options for birth control while breastfeeding include IUDs, condoms, foam, diaphragms, and cervical caps.