Primary Contraception (BC) refers to methods used regularly to prevent pregnancy, such as the pill, patch, ring, injection, implant, or intrauterine device (IUD). Emergency Contraception (EC), like the levonorgestrel pill (Plan B) or ulipristal acetate (Ella), is a backup method intended for use after unprotected sex or contraceptive failure. The necessity of using EC depends entirely on the specific type of primary method being used and the nature of any adherence error that may have occurred.
How Primary Contraception Failure Occurs
The effectiveness of birth control is often reduced by factors related to its use and external influences, not inherent product flaws. The most common reason for failure is user error, such as mismanaging a daily schedule or delaying device reapplication. For methods requiring strict timing, like the oral contraceptive pill, forgetting a dose quickly diminishes the hormone levels needed to suppress ovulation.
External factors can also compromise hormone absorption. Severe vomiting or diarrhea shortly after taking an oral pill prevents the medication from being fully absorbed into the bloodstream. This reduces the contraceptive’s efficacy because the therapeutic dose of hormones is not reached.
Drug interactions are a pharmacologic cause of failure, where certain medications accelerate the metabolism of contraceptive hormones. Drugs that induce the liver enzyme CYP3A4, such as specific antibiotics, anti-seizure medications, or St. John’s Wort, cause the body to break down the hormones too quickly. When hormone concentration levels drop faster than intended, the birth control method may become ineffective.
Situations That Require Emergency Contraception
The decision to use EC hinges on specific, high-risk scenarios where the primary method’s protection is significantly compromised. For users of combined oral contraceptive pills, a major failure is defined as missing two or more consecutive active pills. The risk is highest when pills are missed during the first week of a pack, as this prolongs the hormone-free interval and can allow ovulation to resume.
A similar principle applies to user-dependent hormonal methods like the patch and the vaginal ring. If the patch is detached or the ring is left out for 48 hours or more past its scheduled time, hormone delivery is disrupted and EC should be considered. The user must reapply the device and use a backup barrier method for the next seven days, especially if unprotected sex occurred.
For the contraceptive injection, Depo-Provera, the standard schedule requires an injection every 13 weeks. Protection is maintained for up to 15 weeks from the last injection. If the scheduled injection is administered after this 15-week window, the user is considered at risk, and EC is recommended if unprotected sex occurred. EC is also warranted following the failure of a barrier method, such as a condom breaking, if the user was relying on a primary method that was not yet fully effective.
The effectiveness of emergency contraception is highly time-sensitive, making a prompt decision essential. The levonorgestrel pill (Plan B) is most effective when taken within 72 hours (three days) of the failure, though it can be used up to five days. Ulipristal acetate (Ella), a prescription-only option, is effective for up to 120 hours (five days) and may maintain effectiveness better than levonorgestrel in the later hours of this window.
Birth Control Types Where Plan B Is Not Needed
In many situations, emergency contraception is unnecessary or may be less effective than the primary method already in use. Long-Acting Reversible Contraceptives (LARCs), including the hormonal implant and all types of IUDs, are the most effective primary methods available. These methods have a perfect-use failure rate of less than 1% and are largely immune to user error, absorption issues, or typical drug interactions.
A person using a LARC generally does not need emergency contraception unless the device has been expelled or its placement is incorrect. For users of the combined oral contraceptive pill, a single missed pill taken within 48 hours of the scheduled time is not considered a failure requiring EC. In this case, taking the missed pill immediately and continuing the pack as normal is sufficient to maintain protection.
The copper IUD is the single most effective form of emergency contraception, preventing over 99% of pregnancies when inserted within five days of unprotected sex. A person who already has a copper IUD in place has the highest possible level of protection and would not need to take a hormonal EC pill like Plan B. Furthermore, ulipristal acetate (Ella) can reduce the effectiveness of progestin-containing hormonal birth control, so the choice of EC must be carefully considered based on the ongoing primary method.
Post-Emergency Contraception Follow-Up
After taking emergency contraception, specific steps are necessary to ensure continued pregnancy prevention. Users should immediately consult a healthcare provider to determine the best course of action for continuing or restarting their primary birth control method. If the user took the levonorgestrel pill, they can generally resume their regular hormonal method right away.
If ulipristal acetate (Ella) was used, the user must wait five days before starting or resuming any hormonal contraceptive, as the EC medication can interfere with the primary method’s hormones. Regardless of the EC type, a backup barrier method, such as condoms, should be used for at least seven days until the primary contraceptive is fully effective again.
Emergency contraception can alter the timing of the next menstrual period, sometimes causing a delay of up to a week. If a period is missed entirely or is late by more than seven days, a pregnancy test should be taken three weeks after the EC was used. This follow-up confirms the emergency measure was successful and allows for a review of the primary birth control method to prevent future failures.