Selecting Emergency Contraception by Timing and Cycle

The best emergency contraceptive depends on the hours since intercourse and on the position of the episode in the cycle. Ovulation usually occurs about 14 days before the next menses, so the fertile window in a regular 28-day cycle is roughly cycle days 9 to 14. One act in the second or third week carries about 8 pregnancies in 100 women, and oral emergency contraception reduces this to 1 to 2 in 100.

Why timing matters.

• Levonorgestrel works only when taken before the luteinising hormone surge starts. It does not prevent ovulation once the surge is under way.

• Ulipristal acetate delays ovulation even after the surge has started, up to shortly before the peak.

• The copper IUD acts at the level of sperm and ovum and does not depend on the surge. It is effective until implantation, which allows insertion up to 5 days after the earliest expected ovulation.

• After ovulation, oral options do not prevent pregnancy from that episode, but they are not harmful.

Efficacy by interval. All oral options work best in the first 24 hours and lose effect as hours pass. Ulipristal acetate keeps its effect better to 120 hours. The copper IUD does not lose effect within its window.

Return to fertility. Immediate. Oral doses delay ovulation, so protection does not extend to later acts in the cycle.

Key history. Hours since each episode; number of episodes in the cycle; first day of the last menstrual period; cycle length and regularity; whether menses have since started; hormonal method in use or recently stopped; breastfeeding and weeks since birth; weight; enzyme inducers.

Eligibility. The categories are unchanged by timing. Category 1 applies to breastfeeding with levonorgestrel, previous ectopic pregnancy, sexual assault, obesity and enzyme inducers. Category 2 applies to ulipristal acetate in breastfeeding, cardiovascular disease, angina, migraine and severe liver disease.

Special cycle states.

• Irregular cycles or no recent menses: ovulation timing cannot be predicted, so assume the fertile phase and prefer the copper IUD or ulipristal acetate.

• After a recent hormonal method failure: ovulation may be imminent, so prefer the copper IUD or ulipristal acetate.

• Within 4 weeks after birth: emergency contraception is not needed.

• Amenorrhoea while fully breastfeeding under 6 months: protection holds if all lactational amenorrhoea criteria are met; if any is not met, offer emergency contraception.

Counselling points for the clinician. Explain that the choice reflects efficacy at that time, not preference. Obtain consent and protect confidentiality. Offer condoms for dual protection.

Selection table.

Regimen lines.

• Copper IUD: copper T 380A, inserted once, immediate protection, lasts up to 12 years, no backup.

• Ulipristal acetate: 30 mg by mouth as one dose, within 120 hours. No progestin for 5 days after.

• Levonorgestrel: 1.5 mg by mouth as one dose, within 120 hours. Single dose of 3 mg with enzyme inducers when nothing else is possible.

Pregnancy exclusion. Pregnancy is reasonably excluded when there has been no unprotected intercourse since the last normal menses and menses are not overdue. A test is not needed before one recent episode. Quick start of ongoing contraception with backup is acceptable after emergency contraception.

Vomiting. Within 2 hours of an oral dose, repeat it with an antiemetic. After more than 2 hours, give nothing further.

Follow up. Test 3 weeks after the episode if menses have not started, or earlier if menses are more than 7 days late.

Failure patterns. Oral doses fail most often when taken after the surge, when delayed beyond 72 hours, at higher body weight and with enzyme inducers. A patient who has intercourse again after a dose in the same cycle is not covered.

Management of failure risk. Offer a second dose or the copper IUD for the new episode. Start a regular method at once.

Effects of the options. Nausea, headache, abdominal pain and altered menses. Menses may be early or late by a few days.

Antiemetic regimen. Ondansetron 4 mg by mouth as one dose, repeated once after 8 hours if needed. Maximum 12 mg in 24 hours, for no more than 2 days. Limit to 8 mg in 24 hours in severe liver disease. No kidney adjustment is needed. Avoid in congenital long QT syndrome. In pregnancy use only when the benefit is clear. Short courses are acceptable in breastfeeding.

Common errors. Offering levonorgestrel only at 120 hours, missing the copper IUD window after ovulation, and starting a progestin within 5 days after ulipristal acetate.

Referral and escalation. Refer for severe lower abdominal pain, fainting or a positive pregnancy test with pain or bleeding.