On which day of the menstrual cycle should the contraceptive pill, specifically combined oral contraceptives (COCs) or progestin-only pills (POPs), be started?

When to Start the Contraceptive Pill

Combined oral contraceptives (COCs) and progestin-only pills (POPs) can be started at any time during the menstrual cycle if you are reasonably certain the woman is not pregnant, but starting within the first 5 days of menstrual bleeding eliminates the need for backup contraception. 1

Combined Oral Contraceptives (COCs)

Optimal Timing for Starting

• If started within the first 5 days of menstrual bleeding, no backup contraception is needed 2
• If started more than 5 days after menstrual bleeding begins, use backup contraception (condoms) or avoid intercourse for the first 7 days 2
• COCs can be initiated at any point in the cycle if pregnancy is reasonably excluded, though backup protection requirements vary 1, 3

Special Clinical Situations

Amenorrhea (not postpartum):

• Start at any time if pregnancy is excluded 1
• Backup contraception required for 7 days 2

Postpartum (not breastfeeding):

• Can start immediately postpartum 1
• If ≥21 days postpartum and menses have not returned, use backup contraception for 7 days 2

Postpartum (breastfeeding):

• Delay estrogen-containing methods until 6 weeks to 6 months postpartum 3
• If <6 months postpartum, amenorrheic, and fully/nearly fully breastfeeding, no backup needed 1

Post-abortion:

• Can start immediately, including at time of surgical abortion 1

Progestin-Only Pills (POPs)

Timing Requirements

• If started within the first 5 days of menstrual bleeding, no backup contraception needed 1
• If started >5 days after menstrual bleeding began, abstain from intercourse or use backup contraception for 2 days 1
• POPs can be initiated at any time if pregnancy is reasonably excluded 1

Key Differences from COCs

• POPs require only 2 days of backup contraception (versus 7 days for COCs) when started late in the cycle 1, 4
• Can be started immediately postpartum regardless of breastfeeding status 1
• Desogestrel-containing POPs show contraceptive effectiveness similar to COCs 5

Evidence on Cycle Day Initiation

Research demonstrates that starting COCs on different cycle days does not increase pregnancy rates, though follicular activity increases with later initiation 6. No ovulations occurred when COCs were started at follicle diameter ≤10 mm (approximately cycle day 7-8), supporting the safety of early cycle initiation 6.

Common Pitfall to Avoid

Do not delay contraceptive initiation waiting for the next menstrual period when switching methods or starting for the first time—this increases risk of unintended pregnancy 1, 3. The "Quick Start" approach (starting on the day of the office visit) is safe and may improve initial continuation, though long-term continuation rates equalize 6.