Nexplanon is one of the most effective birth control methods available, with a real-world pregnancy rate of roughly 0.02 per 100 women per year. That translates to about 1 in 5,000 users becoming pregnant annually while the implant is in place. For context, it’s more effective than the pill, the patch, condoms, and even sterilization procedures like tubal ligation.
The Numbers Behind Nexplanon’s Effectiveness
Under perfect laboratory conditions, earlier studies found the implant’s failure rate to be essentially zero. In real-world use, where timing, body chemistry, and medications all play a role, the rate rises slightly. The NORA study (Nexplanon Observational Risk Assessment Study), which tracked thousands of everyday users, calculated a Pearl Index of 0.02, meaning 2 pregnancies per 10,000 woman-years of use. When pregnancies occurring within 7 days of removal were included, that number rose to 0.04.
To put that in perspective: the birth control pill has a typical-use failure rate of about 7 out of 100 women per year, mostly because people miss doses. Nexplanon sits in your arm and releases a steady stream of hormone, so there’s virtually no room for user error. Among reversible contraceptives, it’s the single most effective option. It rivals permanent sterilization procedures, which also carry a small but nonzero failure rate.
How Nexplanon Prevents Pregnancy
The implant works primarily by stopping ovulation. Within 8 hours of insertion, it suppresses the hormonal surge your body needs to release an egg. No egg, no fertilization. As a backup, the hormone also thickens cervical mucus, making it harder for sperm to reach the uterus, and thins the uterine lining, making implantation less likely. These layered mechanisms are why the failure rate is so low.
Why Pregnancies Still Happen
Although rare, pregnancies on Nexplanon do occur. The most well-documented cause is drug interactions. Certain medications speed up the rate at which your liver breaks down the implant’s hormone, lowering its concentration in your blood. The FDA lists several categories of medications that can do this:
- Seizure medications such as carbamazepine, phenytoin, oxcarbazepine, topiramate, and felbamate
- Antibiotics and antifungals such as rifampin and griseofulvin
- HIV antiretrovirals, including certain protease inhibitors
- Herbal supplements, specifically St. John’s wort
If you take any of these long-term, the FDA recommends switching to a contraceptive method that isn’t affected by these interactions. A short course of one of these drugs may call for backup contraception like condoms, but a conversation with your prescriber is the best way to sort that out.
Other possible reasons for failure include an implant that wasn’t placed correctly or, in very rare cases, an implant that goes undetected after it migrates from its original position in the arm. These scenarios are uncommon but have been reported.
Does Body Weight Affect How Well It Works?
This is a reasonable concern, and the answer has evolved over time. When Nexplanon was first approved, the clinical trial excluded women above 130% of their ideal body weight. Because of that gap in data, the label carried a warning about potentially reduced effectiveness in heavier women.
A more recent FDA-reviewed study specifically enrolled overweight and obese participants, who made up 38% of the study group and contributed over 2,500 menstrual cycles of data. The results showed consistent efficacy across all BMI categories, ages, and racial groups. However, because that study only lasted two years rather than the full three-year lifespan of the implant, the FDA has been cautious about fully removing the weight-related warning from the label. In practical terms, current evidence suggests the implant works well regardless of weight, but the question of whether effectiveness drops slightly in the third year for higher-weight users hasn’t been definitively settled.
Timing Your Insertion Matters
When Nexplanon is placed within the first 5 days of your period, it’s effective immediately. If it’s placed at any other point in your cycle, you’ll need to use condoms or avoid intercourse for the next 7 days while the hormone reaches effective levels. Missing this window is one of the few user-dependent factors that could leave a gap in protection.
Side Effects That Mimic Pregnancy
Many Nexplanon users experience side effects that feel eerily similar to early pregnancy symptoms, which can cause unnecessary anxiety. Breast tenderness, nausea, mood changes, and weight gain are all reported side effects of the implant itself. Perhaps the most alarming overlap is that Nexplanon can stop your period entirely, a condition called amenorrhea. Missing your period when you’re on the implant is common and expected, not a sign that something has gone wrong.
That said, if you’re experiencing these symptoms alongside a missed period and have reason to believe the implant might not be working (for example, you’re taking one of the interacting medications listed above, or you can’t feel the implant in your arm), a home pregnancy test will give you a reliable answer. The implant does not interfere with pregnancy test accuracy.
How Long the Protection Lasts
Nexplanon is FDA-approved for 3 years. After that, hormone levels may drop low enough to allow ovulation. Researchers have been studying whether the implant remains effective beyond three years, with a major clinical trial randomizing users to either keep their implant or replace it at the three-year mark and then tracking outcomes for up to three additional years. Results from that trial could eventually change guidance, but for now, replacing the implant at three years is the standard recommendation.
Once removed, fertility returns quickly. Ovulation typically resumes within 3 to 4 weeks, so if you’re not planning a pregnancy, you’ll want a new method in place right away.