It is typically not recommended for women who are breastfeeding to use contraception that contains estrogen. Research on the safety of using this hormone while nursing has shown that it can decrease breast milk production.
In addition, if a woman uses estrogen immediately after childbirth, regardless if she is breastfeeding, it can increase the risk of developing a serious blood clot. Therefore, it is not usually recommended for women to use birth control medications containing estrogen within the four weeks after childbirth.
If a woman wants to use hormonal contraception while breastfeeding, it is typically considered safe to use a progestin-only form of birth control. Progestin products are considered safe for women and children, and do not decrease breast milk production. Women can start taking these products safely six weeks after childbirth. Some progestin-only products include pills, injections, and implants.
(Click Breastfeeding and Birth Control for more details on the safety of using hormonal contraception while breastfeeding. This eMedTV article also describes other birth control options that are safe to use while nursing.)