Nexplanon prevents pregnancy through three overlapping mechanisms: it stops ovulation, thickens cervical mucus so sperm can’t reach an egg, and thins the uterine lining to discourage implantation. The implant is a small, flexible rod inserted under the skin of your upper arm, where it steadily releases a synthetic progestin into your bloodstream for up to five years. With a real-world failure rate of roughly 0.02 pregnancies per 100 women per year, it is the most effective reversible contraceptive available.
Stopping Ovulation Is the Primary Effect
The implant’s main job is preventing your ovaries from releasing an egg each month. It does this by delivering a steady stream of synthetic progestin into your bloodstream, which signals your brain to suppress the hormonal surge that normally triggers ovulation. Without that surge, the ovary never gets the green light to release a mature egg, and fertilization simply can’t happen.
The hormone release isn’t constant over the implant’s lifespan. In the first several weeks, the rod releases about 60 to 70 micrograms per day. That drops to roughly 35 to 45 micrograms by the end of year one, 30 to 40 micrograms by the end of year two, and 25 to 30 micrograms by the end of year three. Even at these lower levels, the dose remains high enough to reliably block ovulation. In a clinical trial that followed 399 women using the implant into years four and five, zero pregnancies occurred.
How Cervical Mucus Creates a Barrier
Even if ovulation were somehow to slip through, Nexplanon has a backup. The progestin thickens the mucus at the opening of the cervix, turning it into a dense, sticky plug. Normally, cervical mucus thins around ovulation to help sperm travel into the uterus. With the implant in place, the mucus stays thick throughout the cycle, making it extremely difficult for sperm to swim past the cervix and reach an egg.
Changes to the Uterine Lining
The third layer of protection involves the endometrium, the tissue lining the inside of the uterus. In a typical cycle, this lining thickens and becomes rich with blood vessels to support a fertilized egg. Nexplanon keeps the lining thin and inactive, with only weak cell activity rather than the robust growth needed for a pregnancy to take hold. Research on the implant’s endometrial effects describes the tissue as thin but not atrophic, showing primarily inactive or weak proliferation. This makes the uterine environment unfavorable for implantation in the unlikely event an egg were fertilized.
These endometrial changes are also why many people with the implant experience lighter periods or irregular bleeding. The lining simply doesn’t build up the way it normally would.
How Quickly It Starts Working
Timing matters. If the implant is inserted during the first five days of your menstrual period (or during the placebo week of birth control pills), it starts working right away. If it’s placed at any other point in your cycle, you’ll need to use condoms or another barrier method for seven days while the hormone reaches effective levels in your bloodstream.
Why It’s So Effective
Most contraceptives have a gap between “perfect use” and “typical use” effectiveness because they rely on you to do something correctly every time, whether that’s taking a pill at the same hour, applying a patch, or using a condom. Nexplanon largely eliminates human error. Once the rod is in your arm, there’s nothing to remember, nothing to replace daily, and no way to use it incorrectly.
A large observational study tracking real-world users found a Pearl Index of 0.02 for pregnancies that occurred while the implant was in place, meaning about 2 pregnancies per 10,000 women per year. Even when including pregnancies that happened within a week of removal, the rate was just 0.04. Previous research has placed the upper bound of the failure rate at 0.06 per 100 woman-years. For context, the typical-use failure rate for birth control pills is around 7 per 100 women per year.
Medications That Can Reduce Effectiveness
Certain drugs speed up the liver’s processing of hormones, which can lower the amount of progestin circulating in your blood and potentially weaken the implant’s protection. These include some seizure medications (such as those containing phenytoin, carbamazepine, or topiramate), the antibiotic rifampicin, the antifungal griseofulvin, and the herbal supplement St. John’s wort. If you take any of these, a non-hormonal backup method is recommended while you’re on the medication and for 28 days after stopping it.
What Happens After Removal
Once the implant is taken out, its effects reverse quickly. Fertility can return almost immediately, and it’s possible to become pregnant within days of removal. The progestin clears from the bloodstream rapidly because it was being delivered continuously rather than stored in the body. If you’re having the implant removed and don’t want to become pregnant, you’ll need another form of contraception right away.