An emergency contraception question is often a timing question disguised as pharmacology. If intercourse occurred 48 hours ago and the patient now has a positive urine LH test, the high-yield issue is not merely whether a pill is still within a five-day window. It is whether an oral method can still act before ovulation—or whether the patient needs a method that remains effective after ovulation but before implantation.
The physiologic target: separate sperm from ovulation
Oral emergency contraception works mainly by delaying or inhibiting ovulation, typically by postponing follicular rupture. That delay matters because viable sperm may remain in the upper genital tract for several days. If ovulation is delayed until sperm are no longer capable of fertilization, pregnancy is prevented.
Once ovulation has already occurred, oral emergency contraception has no reliable demonstrated ability to prevent pregnancy from that intercourse. Neither levonorgestrel nor ulipristal acetate interrupts an established pregnancy; these drugs are not medications for terminating an established pregnancy.
The copper intrauterine device is different. It does not need to suppress the ovary. Copper ions impair sperm motility and function, while the local intrauterine environment also affects ova and can prevent implantation if fertilization occurs. Its primary action is before fertilization, but it also has a postfertilization effect; this is why it can remain effective when inserted after ovulation but before implantation, within the accepted insertion window.
The board-level distinction is not simply hormonal versus nonhormonal. It is whether ovulation is still modifiable, or whether the patient needs a method that acts independently of ovarian suppression.
Why levonorgestrel and ulipristal are not interchangeable
| Method | Main biologic target | Clinical consequence |
|---|---|---|
| Levonorgestrel | Progestin-mediated delay or suppression of the LH surge and follicular rupture | Most useful before the LH surge; effectiveness falls as ovulation approaches |
| Ulipristal acetate | Selective progesterone-receptor modulation with inhibition of follicular rupture | Can still delay rupture after the LH rise begins but before the LH peak |
| Copper IUD | Copper-related impairment of sperm and ova in the reproductive tract | Does not depend on delaying ovulation; the only EC method known to remain effective after ovulation when inserted within the accepted window |
Levonorgestrel is a progestin. Given sufficiently early, it can alter hypothalamic-pituitary-ovarian signaling, blunt the LH surge, and delay ovulation. Its limitation is timing: once the dominant follicle has entered the LH surge, a single levonorgestrel dose is much less able to prevent follicular rupture.
Ulipristal acetate is a selective progesterone receptor modulator, not simply a higher-dose version of levonorgestrel. Pharmacodynamic studies show that it can delay follicular rupture after the LH surge has begun, provided the LH peak and ovulation have not already occurred. That is why ulipristal is generally the stronger oral option when intercourse occurred several days earlier or appears to have occurred during the fertile interval.
The distinction is clinically important but should not be overstated. Ulipristal is not guaranteed to prevent ovulation after the LH peak. A positive home urine LH test also does not provide a precise timestamp: it signals changing LH physiology, but it cannot reliably prove whether the peak or ovulation has already passed.
Turning mechanism into a clinical choice
The initial history should establish more than the number of hours since intercourse. Ask about the exact timing of each episode of unprotected intercourse, cycle regularity, the possibility of recent ovulation, recent use of hormonal contraception, use of enzyme-inducing medications, body weight or BMI, and whether the patient wants ongoing intrauterine contraception.
Cycle-day calculations and fertility applications are clues, not direct measurements of ovulation. A patient with irregular cycles, recent emergency contraception, postpartum amenorrhea, or recently discontinued hormonal contraception may have particularly unreliable calendar estimates.
When the copper IUD deserves priority
The copper IUD is the most effective emergency contraception and provides immediate ongoing contraception. In U.S. guidance, it may be placed within five days of the first act of unprotected intercourse; when the timing of ovulation can be estimated, placement may also occur more than five days after intercourse as long as it is no more than five days after ovulation.
This makes the copper IUD the key option when ovulation may already have occurred. It is the only emergency contraceptive method known to remain effective after ovulation, provided it is placed before implantation and within the appropriate window. It also avoids concerns about weight-related reduction in oral emergency-contraception effectiveness, particularly with levonorgestrel, and about enzyme-inducing drug interactions. The decision still requires assessment of IUD eligibility, patient preference, infection risk, and access to timely insertion.
When an oral option is appropriate
If an IUD is not acceptable, unavailable, or clinically unsuitable, oral selection should reflect both elapsed time and likely cycle position.
Ulipristal is particularly useful when intercourse occurred 3–5 days earlier or may have occurred during the five days before ovulation. Levonorgestrel remains a reasonable option when taken promptly, especially before the LH surge, when ulipristal is unavailable or unsuitable, or when immediate restart of hormonal contraception is a major priority. Enzyme-inducing drugs can reduce oral emergency-contraception effectiveness; when feasible, a copper IUD avoids this interaction.
The difference in follow-up is mechanistically important. After ulipristal, progestin-containing contraception should generally be started or resumed no sooner than five days later because progestin exposure may compete with ulipristal at the progesterone receptor and reduce its ability to delay ovulation. For provider-administered methods such as depot medroxyprogesterone acetate, an implant, or a hormonal IUD, same-day initiation may be considered case by case, weighing possible reduction in ulipristal efficacy against the risk of delaying ongoing contraception. After levonorgestrel, regular hormonal contraception can be started or resumed immediately. In either case, barrier protection is needed until the ongoing method becomes effective.
Neither oral option provides continuing protection against new intercourse later in the cycle. If there is no expected withdrawal bleed or menses, a pregnancy test is appropriate three weeks after the most recent episode of unprotected intercourse.
Common board traps
- All oral emergency contraception works equally well until 120 hours. False. Ulipristal retains better efficacy later in the five-day interval; levonorgestrel is more time-sensitive.
- A positive LH test means oral emergency contraception is useless. Too absolute. Levonorgestrel is less reliable once the LH surge is underway, while ulipristal may still delay follicular rupture before the LH peak.
- Emergency contraception prevents implantation as its primary action. For the oral methods, this is an outdated simplification: evidence supports delaying or inhibiting ovulation as their primary mechanism, and clinically important postovulatory efficacy has not been demonstrated.
- The copper IUD works by suppressing ovulation. It does not. Its major action is local interference with sperm and fertilization, with additional postfertilization effects.
- Taking ulipristal and immediately restarting a progestin method is harmless. The timing matters because progestin may diminish ulipristal’s ovulation-delaying effect.
Practical takeaways
- Start with the biologic question: has ovulation probably occurred, or is it still potentially modifiable—and, if it has occurred, is a copper IUD still within its insertion window?
- Levonorgestrel works best before the LH surge and becomes less reliable as ovulation approaches.
- Ulipristal can delay follicular rupture after the LH rise begins but before the LH peak; it is not reliably effective after ovulation.
- The copper IUD is the most effective option and the only emergency-contraception method known to remain effective after ovulation when placed before implantation within the accepted insertion window.
- After ulipristal, delay progestin-containing contraception for five days unless a selected provider-administered method is started after weighing the tradeoffs; after levonorgestrel, hormonal contraception can begin immediately.
- Oral emergency contraception covers prior intercourse, not later intercourse in the same cycle.