Levonorgestrel (LNG) is a progestin-only medication widely used as an emergency contraceptive (EC) following unprotected sexual intercourse or contraceptive failure. This hormonal preparation is intended as a backup method to significantly reduce the chance of unintended pregnancy. It is typically administered as a single 1.5 mg dose and is most effective the sooner it is taken. The medication acts to interrupt the natural biological process that leads to pregnancy.
How Levonorgestrel Prevents Conception
The primary action of levonorgestrel is to interfere with the complex signaling pathway that controls the monthly release of an egg. LNG acts as a synthetic progestin, introducing a high dose of hormone that disrupts the normal signals between the brain and the ovaries. The medication targets the hypothalamic-pituitary-ovarian axis, which regulates the menstrual cycle. By doing so, LNG suppresses the pre-ovulatory surge of Luteinizing Hormone (LH) necessary for a mature egg to be released from the follicle.
This suppression of the LH surge prevents the rupture of the ovarian follicle, which is the physical process of ovulation. If taken before the LH surge begins, the medication effectively blocks the signal to ovulate, preventing the egg and sperm from meeting. The drug is highly effective when administered in the pre-ovulatory phase of the cycle. However, levonorgestrel is not effective once the LH surge is already underway or if ovulation has already occurred, as its mechanism is preventive.
The Specific Timeframe of Ovulation Delay
The duration of the ovulation delay caused by levonorgestrel is not fixed; it depends entirely on the timing of administration relative to the menstrual cycle. The goal of the medication is to delay the egg’s release long enough for any sperm present in the reproductive tract to become non-viable, which occurs within five days. If LNG is taken several days before the expected LH surge, it can delay the surge and subsequent ovulation significantly.
Clinical data demonstrates that when administered in the early-to-mid follicular phase, levonorgestrel can postpone the LH peak and the onset of the next menses by approximately five to seven days on average. In some cases, the delay has been observed to be much longer, with the LH peak being delayed by over two weeks compared to an untreated cycle. This variability highlights the medication’s direct impact on the timing of the hormonal cascade. Effectiveness drops sharply if the medication is taken very close to the natural LH surge, as the hormonal cascade leading to ovulation cannot be halted.
The window of effectiveness is narrow because levonorgestrel must be present in high concentrations to block the signal before the final stages of follicular development are complete. Women who took the drug three days before their expected ovulation experienced a significant delay in their LH peak, while those who took it just one day before saw no significant delay. Therefore, the ability to delay ovulation is directly tied to the stage of the ovarian cycle at the time of ingestion, emphasizing the importance of taking it as soon as possible.
Return to Normal Cycle Timing
Following the temporary hormonal disruption caused by levonorgestrel, the body’s natural cycle timing will resume. The most immediate effect is a change in the timing of the next menstrual period. The next period may arrive earlier or later than the expected date, and it is normal for the start date to shift by up to one week in either direction.
If the next period is delayed by more than seven days past its expected start date, a pregnancy test should be taken to rule out contraceptive failure. The high dose of progestin can also affect bleeding characteristics, potentially causing the flow to be lighter or heavier than usual, or resulting in unexpected spotting before the next true period. These transient changes in the menstrual pattern typically resolve completely in the cycle immediately following LNG use.
The effect of levonorgestrel is limited to the single act of unprotected intercourse that preceded its use. Fertility returns immediately once the drug is metabolized and clears the system, meaning there is no residual contraceptive effect. Subsequent acts of unprotected intercourse, even within the same cycle, carry the same risk of pregnancy and require additional contraceptive protection.