21 days

Amount of time women who have just given birth should wait before using birth control pills, because of the high risk of potentially fatal blood clots. Women with other risk factors for blood clots — those who delivered by Cesarean section, are obese or have a history of blood clots — should wait six weeks before starting the pills. Using contraception is crucial during the postpartum period, especially for new mothers who do not breast-feed and may therefore ovulate as soon as 25 days after giving birth, but women must be aware the risks of combined hormonal birth control. [via CDC]