Understanding Plan B and Its Primary Function
Plan B One-Step is a form of emergency contraception, often called the "morning-after pill" [1.2.2]. Its active ingredient is levonorgestrel, a synthetic hormone found in lower doses in many regular birth control pills [1.3.4]. The main purpose of Plan B is to prevent pregnancy after unprotected sex or contraceptive failure [1.5.4]. It is most effective when taken as soon as possible, within 72 hours (3 days) of the event [1.2.7].
The primary mechanism of action for Plan B is the prevention or delay of ovulation [1.3.4]. It works by inhibiting the surge of luteinizing hormone (LH), which is the hormonal trigger that causes an ovary to release a mature egg [1.5.1, 1.6.3]. By stopping this LH surge, Plan B ensures there is no egg available for sperm to fertilize [1.5.2]. Sperm can live in the female reproductive tract for up to five days, so delaying the release of an egg past this window prevents conception [1.2.1, 1.5.7].
The Critical Timing: Does Plan B Work If Ovulation Is Happening?
The effectiveness of Plan B is almost entirely dependent on its timing in relation to a person's menstrual cycle [1.2.1]. If you take Plan B before the LH surge has begun, it is highly effective at preventing ovulation [1.6.2]. However, if the LH surge has already started or if ovulation (the release of the egg) is in progress or has already happened, Plan B will not be effective [1.2.1, 1.2.5, 1.6.3]. Once the egg is released, the pill cannot stop it from being fertilized by sperm [1.5.2].
Studies have shown that when levonorgestrel is taken on the day of ovulation or afterward, it does not significantly reduce the number of expected pregnancies [1.4.3]. This confirms that its action is pre-ovulatory. Because it can be very difficult for a person to know the exact moment of ovulation, experts often recommend taking emergency contraception as soon as possible regardless of where you think you are in your cycle [1.2.1].
Common Misconceptions
It's a common misconception that Plan B works by preventing a fertilized egg from implanting in the uterus [1.3.4]. However, the majority of scientific research indicates that levonorgestrel does not significantly alter the lining of the uterus (endometrium) and therefore does not interfere with implantation [1.3.4, 1.4.7]. Plan B is not an abortion pill; it does not terminate an existing pregnancy and will not harm a developing fetus if taken inadvertently when already pregnant [1.2.3, 1.2.6].
Comparison of Emergency Contraception Options
When Plan B may not be effective, particularly around the time of ovulation, other options are available. The effectiveness of different types of emergency contraception (EC) varies based on their mechanism and the timing of use.
| Feature | Levonorgestrel (Plan B, Julie) | Ulipristal Acetate (ella) | Copper IUD (Paragard) |
|---|---|---|---|
| Primary Mechanism | Delays or prevents ovulation [1.3.4] | Delays ovulation, even after LH surge begins [1.7.5] | Prevents fertilization by being toxic to sperm; can prevent implantation [1.2.2, 1.2.7] |
| Effectiveness Window | Up to 72 hours (3 days) [1.2.7] | Up to 120 hours (5 days) [1.7.4] | Up to 120 hours (5 days) [1.2.1] |
| Effectiveness During/After Ovulation | Ineffective [1.2.1, 1.2.2] | More effective than Plan B closer to ovulation, but not after [1.7.5] | The most effective option, works even after ovulation [1.2.7, 1.4.6] |
| Prescription Required | No [1.7.4] | Yes [1.7.4] | Yes, requires insertion by a clinician [1.2.1] |
| BMI Considerations | May be less effective in people over 165 lbs [1.7.3] | May be less effective in people over 195 lbs [1.7.3] | Not affected by weight [1.7.3] |
What to Expect After Taking Plan B
After taking Plan B, it is common to experience some side effects due to the high dose of hormones. These can include:
- Nausea or vomiting [1.2.4]
- Headache, dizziness, or fatigue [1.2.4]
- Breast tenderness [1.2.4]
- Lower abdominal cramping [1.2.4]
- Changes to your next menstrual period [1.3.4]
Your next period may come earlier or later than expected, and the flow may be heavier or lighter than usual [1.5.5]. Some people may also experience spotting a few days after taking the pill [1.5.5]. If your period is more than a week late, it is recommended to take a pregnancy test [1.5.5].
Conclusion
The functionality of Plan B is clear: it works by delaying ovulation. Therefore, if you are already ovulating or have just ovulated, Plan B will not be an effective method for preventing pregnancy. Its success is entirely dependent on being taken before the egg is released. Given the difficulty in pinpointing the exact time of ovulation, taking it as soon as possible after unprotected sex provides the best chance for it to work. If you believe you may be ovulating or have already ovulated, other forms of emergency contraception, like ulipristal acetate (ella) or the copper IUD, are more effective options [1.2.6, 1.7.5].
For more information on contraceptive options, consider visiting a reputable health resource like Planned Parenthood..
