Nexplanon is the most effective reversible contraceptive available, and pregnancies on it are extremely rare. In the largest prospective study of the implant (the NORA study), only 3 pregnancies occurred while the device was actually in place, yielding a failure rate of about 0.02 per 100 women per year. To put that in practical terms, fewer than 1 in 5,000 users per year becomes pregnant while the implant is working as intended.
That number is low enough that many people wonder if it’s even real. It is. But the rare pregnancies that do happen tend to follow a pattern, and understanding why they occur can help you make sense of the risk.
What the Studies Actually Found
The NORA study tracked thousands of Nexplanon users in real-world conditions. Six total pregnancies were recorded: 3 occurred while the implant was in place, and 3 happened within 7 days of having it removed. When all six are counted, the Pearl Index (the standard measure of contraceptive failure) was 0.04 pregnancies per 100 woman-years of use. When only the during-use pregnancies are counted, it drops to 0.02.
A separate FDA extension study looked at 394 women who kept the implant in for a fourth and fifth year, well beyond the originally approved 3-year window. Over nearly 7,000 evaluable menstrual cycles, zero pregnancies occurred. The failure rate for years 4 and 5 combined was literally 0.0 per 100 woman-years. This data contributed to the FDA’s recent approval of Nexplanon for up to 5 years of use.
How That Compares to Other Birth Control
Nexplanon’s effectiveness sits at the very top of the contraceptive hierarchy. IUDs come close, with failure rates also under 1%, but the implant edges them out slightly in most head-to-head comparisons. The gap widens dramatically when you look at methods that depend on daily habits. Birth control pills are theoretically 99% effective, but in practice only about 91% effective because people miss doses, take them late, or stop and restart. That 9% real-world failure rate means roughly 9 out of 100 pill users will become pregnant in a given year.
Nexplanon doesn’t have a “typical use” vs. “perfect use” gap. Once it’s in your arm, there’s nothing you need to do. You can’t forget to take it, use it incorrectly, or accidentally skip a day. That’s what makes it so reliable.
Why the Rare Pregnancies Happen
When pregnancies do occur on Nexplanon, they almost always trace back to one of a few specific causes rather than the implant simply failing on its own.
The Implant Wasn’t Actually in Place
The most straightforward reason: the device was never properly inserted. If the implant doesn’t deploy into the tissue of your arm during the insertion procedure, you have no contraceptive protection at all. This is why your provider should have you feel the implant under your skin immediately after placement, and why the FDA label states you should use condoms until the implant’s presence has been confirmed.
Drug Interactions
Certain medications speed up the way your liver breaks down hormones, which can lower the implant’s hormone levels enough to allow ovulation. The most well-documented culprits include some anti-seizure medications (phenytoin, carbamazepine, topiramate, oxcarbazepine), the antibiotic rifampicin (used for tuberculosis), and the herbal supplement St. John’s wort. Several HIV medications can also decrease the implant’s hormone levels. Researchers reviewing implant failures in women with higher body weight concluded that these drug interactions were more likely to explain the pregnancies than weight itself.
Timing of Insertion
If Nexplanon is placed during the first 5 days of your period, it’s effective immediately. If it’s inserted at any other point in your cycle, you need to use a backup method like condoms for 7 days. A pregnancy that seems to have occurred “on Nexplanon” may actually have resulted from unprotected sex during that initial 7-day window.
Does Body Weight Affect How Well It Works
This is one of the most common concerns, and the evidence is reassuring. A systematic review looking specifically at implant effectiveness in women with overweight and obesity found that failure rates in this group ranged from 0.0 to 0.23 per 100 woman-years. That falls squarely within the range seen across all weight groups (0.0 to 1.4 per 100 woman-years). The researchers concluded that body weight does not appear to reduce the implant’s effectiveness in a meaningful way, and that the rare failures in higher-weight users were better explained by enzyme-inducing medications or uncertainty about whether the implant was actually in place.
What This Means in Real Numbers
If 10,000 women use Nexplanon for a year, the data suggests somewhere between 0 and 4 of them will become pregnant, depending on which pregnancies you count. Compare that to the pill, where roughly 900 of those same 10,000 women would become pregnant under typical real-world conditions. The difference is enormous.
Pregnancy on Nexplanon is not impossible, but it is genuinely one of the rarest contraceptive failures in medicine. The few cases that do occur are almost always traceable to a specific, identifiable cause rather than the implant silently stopping work. If you can feel the rod in your arm and you’re not taking medications that interfere with it, your level of protection is about as close to guaranteed as any reversible method can get.